Semiconductor device and electronic apparatus using the same

ABSTRACT

The transistor suffers the variation caused in threshold voltage or mobility due to gathering of the factors of the variation in gate insulator film resulting from a difference in manufacture process or substrate used and of the variation in channel-region crystal state. The present invention provides an electric circuit having an arrangement such that both electrodes of a capacitance element can hold a gate-to-source voltage of a particular transistor. The invention provides an electric circuit having a function capable of setting a potential difference at between the both electrodes of the capacitance element by the use of a constant-current source.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the art of electric circuits.Meanwhile, the invention belongs to a technical field of a semiconductordevice having an electric circuit as represented by a source-followercircuit, a differential amplifier circuit, a sense amplifier and anoperational amplifier, a signal-line drive circuit and a photoelectricconverter element.

2. Description of the Related Art

The integrated circuit (IC), for broad use recently on a cellular phoneor personal digital assistant, is formed with transistors or resistorsas many as several hundreds of thousands to several millions on asilicon substrate in a size of nearly a 5-mm square. This plays animportant role in device miniaturization and reliability improvement,and device mass production.

In designing an electric circuit for use on an integrated circuit (IC)or the like, it is frequent cases to design an amplifier circuit havinga function to amplify a voltage or current of a signal small inamplitude. The amplifier circuit is broadly used because of a circuitrequisite for eliminating strain occurrence to stably operate anelectric circuit.

Herein, explained is the configuration and operation of asource-follower circuit, as one example of amplifier circuit. At first,a configuration example of source-follower circuit will be shown in FIG.5A to explain an operation in a steady state. Next, an operating pointof the source-follower circuit will be explained, by using FIGS. 5B and5C. Finally, an example of source-follower circuit different inconfiguration from FIG. 5A will be shown in FIGS. 6A and 6B, to explainan operation in a transient state.

At first, a steady state operation is explained by using asource-follower circuit in FIG. 5A.

In FIG. 5A, 11 is an n-channel amplifier transistor while 12 is ann-channel bias transistor. Note that, although the amplifier transistor11 and bias transistor 12 in FIG. 5A is of an n-channel type,configuration may be by the use of p-channel transistors. Herein, theamplifier transistor 11 and the bias transistor 12 are assumably thesame in characteristic and size, for simplification sake. It is furtherassumed that the current characteristic of them is ideal. Namely, it issupposed that, even if the amplifier transistor 11 or bias transistor 12is changed in its source-to-drain voltage, there is no change insaturation-region current value.

Meanwhile, the amplifier transistor 11 has a drain region connected to apower line 13 and a source region connected to a drain region of thebias transistor 12. The bias transistor 12 has a source region connectedto a power line 14.

The gate electrode of the bias transistor 12 is applied by a biaspotential V_(b). A power-source potential (high potential power) V_(dd)is applied onto the power line 13 while a ground potential (lowpotential power) V_(ss) (=0V) is applied onto the power line 14.

In the source-follower circuit of FIG. 5A, the gate electrode of theamplifier transistor 11 is made as an input terminal so that an inputpotential V_(in) can be inputted to the gate electrode of the amplifiertransistor 11. Also, the source region of the amplifier transistor 11 ismade as an output terminal so that the potential on the source region ofthe amplifier transistor 11 provides an output potential V_(out). Thegate electrode of the bias transistor 12 is applied by a bias voltageV_(b). When the bias transistor 12 operates in a saturation region, acurrent denoted by Ib assumably flows. At this time, because theamplifier transistor 11 and the bias transistor 12 are in a seriesconnection, the same amount of current flows through the bothtransistors. Namely, when a current Ib flows through the bias transistor12, a current Ib flows also through the amplifier transistor 11.

Herein, determined is an output potential V_(out) in the source-followercircuit. The output potential V_(out) is lower in value than the inputpotential V_(in), by an amount of the gate-to-source voltage V_(gs1) ofthe amplifier transistor 11. At this time, the input potential V_(in),the output potential V_(out) and the gate-to-source voltage V_(gs1) havea relationship satisfying the following Equation (1).V _(out) =V _(in) −V _(gs1)  (1)

In the case the amplifier transistor 11 is operating in the saturationregion, in order to flow a current Ib through the amplifier transistor11 there is a necessity that the gate-to-source voltage V_(gs1) of theamplifier transistor 11 is equal to a bias potential V_(b)(gate-to-source voltage of the bias transistor 12). If so, the followingEquation (2) is held. However, Equation (2) is held only when theamplifier transistor 11 and the bias transistor 12 operate in thesaturation region.V _(out) =V _(in) −V _(b)  (2)

Next explained is an operating point of the source-follower circuit byusing

FIGS. 5B and 5C showing a relationship of between a voltage and acurrent of the amplifier transistor 11 and bias transistor 12. Morespecifically, explanation is made on a case that the gate-to-sourcevoltage V_(gs1) of the amplifier transistor 11 is same in value as thegate-to-source voltage V_(gs2) of the bias transistor 12, by using FIG.5B. Next explained is a case that the gate-to-source voltage V_(gs1) ofthe amplifier transistor 11 is different in value from thegate-to-source voltage V_(gs2) of the bias transistor 12 wherein, forexample, the bias transistor 12 is operating in a linear region, byusing FIG. 5C.

In FIG. 5B, the dotted line 21 shows a relationship between a voltageand a current when the amplifier transistor 11 has a gate-to-sourcevoltage V_(gs1) of V_(b). The solid line 22 shows a relationship betweena voltage and a current when the bias transistor 12 has a gate-to-sourcevoltage V_(gs2) of V_(b). Meanwhile, in FIG. 5C, the dotted line 21shows a relationship between a voltage and a current when the amplifiertransistor 11 has a gate-to-source voltage V_(gs1) of V_(b). The solidline 22 shows a relationship between a voltage and a current when thebias transistor 12 has a gate-to-source voltage V_(gs2) of V_(b)′.

In FIG. 5B, the gate-to-source voltage V_(gs1) of the amplifiertransistor 11 and the gate-to-source voltage V_(gs2) of the biastransistor 12 are in the same value, and further the bias potentialV_(b) and the gate-to-source voltage V_(gs2) of bias transistor 12 arein the same value. Consequently, the gate-to-source voltage V_(gs1) ofthe amplifier transistor 11 is in the same value as the bias potentialV_(b). Namely, this results in V_(gs1)=V_(gs2)=V_(b). The amplifiertransistor 11 and the bias transistor 12 are operating in the saturationregion, as shown in FIG. 5B. At this time, the input potential V_(in)and the output potential V_(out) have a relationship in a linear form.

On the other hand, in FIG. 5C, the gate-to-source voltage V_(gs1) of theamplifier transistor 11 is in a value different from the gate-to-sourcevoltage V_(gs2) of bias transistor 12. Furthermore, the gate-to-sourcevoltage V_(gs2) of bias transistor 12 is in a same value as the biasvoltage V_(b). Meanwhile, it is assumed that the gate-to-source voltageV_(gs1) of the amplifier transistor 11 is at the bias voltage V_(b)′.Namely, this results in V_(gs2)=V_(b) and V_(gs1)=V_(b)′. As shown inFIG. 5C, the amplifier transistor 11 is operating in the saturationregion while the bias transistor 12 is operating in the linear region.At this time, the input potential V_(in), the output potential V_(out)and the bias potential V_(b)′ have a relationship satisfying thefollowing Equation (3).V _(out) =V _(in) −V _(b)′  (3)

Provided that the current flowing upon operating of the bias transistor12 in the linear region is taken Ib′, Ib′<Ib is given. Namely, by havingV_(b)′<V_(b), the both values of the input potential V_(in) and currentIb′ decrease. Thereupon, the bias potential V_(b)′ also decreases. Atthis time, the input potential V_(in) and the output potential V_(out)have a non-linear relationship.

Summarizing the above, in order to increase the amplitude of the outputpotential V_(out) in the source-follower circuit in a steady state, itis preferred to decrease the bias potential V_(b). This is because ofthe following two reasons.

The first reason is that the output potential V_(out) can be increasedat a small bias potential V_(b), as shown in Equation (2). The secondreason is that, in the case of a great bias potential V_(b) value, thebias transistor 12 readily operate in the linear region at a decreasedinput potential V_(in). In case the bias transistor 12 operates in thelinear region, the input potential V_(in) and the output potentialV_(out) are ready to have a non-linear relationship.

Incidentally, because the bias transistor 12 is required in a conductionstate, there is a need to provide a greater value of bias potentialV_(b) than a threshold voltage of the bias transistor 12.

So far explained was the operation in a steady state of thesource-follower circuit. Subsequently, explanation is made on theoperation of the source-follower circuit in a transient state, by usingFIGS. 6A and 6B.

The source-follower circuit shown in FIGS. 6A and 6B has a configurationdesigned by adding a capacitance element 15 to the circuit of FIG. 5A.The capacitance element 15 has one terminal connected to the sourceregion of the amplifier transistor 11 and the other terminal connectedto the power line 16. A ground potential V_(ss) is applied onto thepower line 16.

The capacitance element 15 has a same potential difference at betweenits both electrodes as the output potential V_(out) of thesource-follower circuit. Herein, explained is the operation in a case ofV_(out)<V_(in)−V_(b), by using FIG. 6A. Next explained is the operationin a case of V_(out)>V_(in)−V_(b), by using FIG. 6B.

At first, explanation is made on the operation in a transient state ofthe source-follower circuit in the case of V_(out)<V_(in)−V_(b), byusing FIG. 6A.

In FIG. 6A, when t=0, the gate-to-source voltage V_(gs1) of theamplifier transistor 11 has a greater value than the gate-to-sourcevoltage V_(gs2) of the bias transistor 12. Consequently, a great currentflows through the amplifier transistor 11 to promptly hold charge on thecapacitance element 15. Thereupon, the output potential V_(out)increases to decrease the gate-to-source voltage V_(gs1) value of theamplifier transistor 11.

As time elapses (t=t₁, t₁>0), the amplifier transistor 11 goes into asteady state when its gate-to-source voltage V_(gs1) becomes equal tothe bias potential V_(b). At this time, the output potential V_(out),the input potential V_(in) and the bias potential V_(b) have arelationship satisfying the foregoing Equation (2).

Summarizing the above, in the case of V_(out)<V_(in)−V_(b), thegate-to-source voltage V_(gs1) of the amplifier transistor 11 is greaterin value than the bias potential V_(b). Accordingly, a great currentflows through the amplifier transistor 11, to promptly hold charge onthe capacitance element 15. Hence, the time may be short that isrequired for the capacitance element 15 to hold predetermined charge, inother words the time required in writing a signal to the capacitanceelement 15.

Next, explanation is made on the operation in a transient state of thesource-follower circuit in the case of V_(out)>V_(in)−V_(b), by usingFIG. 6B.

In FIG. 6B, when t=0, the gate-to-source voltage V_(gs1) of theamplifier transistor 11 has a smaller value than the threshold voltageof the amplifier transistor 11. Consequently, the amplifier transistor11 is in a non-conduction state. The charge stored on the capacitanceelement 15 flows in a direction toward the ground potential V_(ss)through the bias transistor 12, finally being discharged. At this time,because the gate-to-source voltage V_(gs2) of the bias transistor 12 isin the same value as the bias potential V_(b), the current flowingthrough the bias transistor 12 is Ib.

As time elapses (t=t₁, t₁>0), the output potential V_(out) decreaseswhile the gate-to-source voltage V_(gs1) of the amplifier transistor 11increases. When the gate-to-source voltage V_(gs1) of the amplifiertransistor 11 becomes equal to the bias potential V_(b), a steady stateis entered. At this time, the output potential V_(out), the inputpotential V_(in) and the bias potential V_(b) have a relationshipsatisfying the foregoing Equation (2). Note that, in the steady state,the output potential V_(out) is kept at a constant value, and chargedoes not flow to the capacitance element 15. Thus, a current Ib flowsthrough the amplifier transistor 11 and bias transistor 12.

Summarizing the above, in the case of V_(out)>V_(in)−V_(b), the time forthe capacitance element 15 to hold predetermined charge, in other wordsthe write time of a signal to the capacitance element 15, relies uponthe current Ib flowing through the bias transistor 12. The current Ibrelies upon a magnitude of the bias potential V_(b). Accordingly, inorder to increase the current Ib and shorten the write time of a signalto the capacitance element 15, a necessity is raised to increase thebias potential V_(b).

Incidentally, as a method of correcting for threshold-voltage variationof a transistor, there is a method that variation is observed by anoutput of a circuit a signal has been inputted and thereafter thevariation is inputted and fed back thereby carrying out a correction(e.g. see Non-Patent Document 1).

[Non-Patent Document] H. Sekine et al, “Amplifier Compensation Methodfor a Poly-Si TFT LCLV with an Integrated Data-Driver”, IDRC' 97, p.45-48.

The foregoing operation of the source-follower circuit is to be carriedout on an assumption the amplifier transistor 11 and the bias transistor12 have the same characteristic. However, for the both transistors,variation occurs in the threshold voltage or mobility due to gatheringof the factors, such as of variation in gate length (L), gate width (W)and gate insulating film thickness or variation in channel-regioncrystal state caused due to the difference in fabrication process orsubstrate used.

For example, it is assumed, in FIG. 5A, that there is variation of 1 Vprovided that the amplifier transistor 11 has a threshold of 3 V and thebias transistor 12 has a threshold of 4 V. If so, in order to flow acurrent Ib, there is a need to apply a voltage for the gate-to-sourcevoltage V_(gs1) of the amplifier transistor 11 lower by 1 V than thegate-to-source voltage V_(gs2) of the bias transistor 12. Namely,V_(gs1)=V_(b)−1 results. If so, V_(out)=V_(in)−V_(gs1)=−V_(in)−V_(b)+1results. Namely, in case variation occurs even by 1 V in the thresholdvoltage of the amplifier transistor 11 and bias transistor 12, variationis also caused in the output potential V_(out).

The present invention has been made in view of the above problems. It isa problem to provide an electric circuit suppressing against theaffection of transistor characteristic variation. More specifically, itis a problem, in an electric circuit having a function of currentamplification, to provide an electric circuit capable of supplying adesired voltage while suppressing against the affection of transistorcharacteristic variation.

SUMMARY OF THE INVENTION

The present invention uses an electric circuit configured as in thefollowing, in order to solve the foregoing problems.

An electric circuit shown in FIG. 3A is configured with a referenceconstant-current source 21, a switching element 22 having a switchingfunction (hereinafter, denoted as SW 22), an n-channel transistor 23 anda capacitance element 24. The transistor 23 has a source regionconnected to a power line 25 and a drain region connected to thereference constant-current source 21. The transistor 23 has a gateelectrode connected to one terminal of the capacitance element 24. Theother terminal of the capacitance element 24 is connected to the powerline 25. The capacitance element 24 has a role to hold a gate-to-sourcevoltage V_(gs) of the transistor 23. Meanwhile, the power line 25 isapplied with a ground potential V_(ss).

In FIGS. 3A-3C, the transistor 23 is assumably of an n-channel type.This, however, is not limitative, i.e. configuration is possible with ap-channel type.

The electric circuit of FIG. 3A sets with a potential difference atbetween the both electrodes of the capacitance element, i.e. agate-to-source voltage of a transistor, such that a current flowingbetween the source and the drain of the transistor is equal to a signalcurrent I_(data) (referred also to as a reference current) caused toflow by the reference constant-current source.

In FIG. 3A, the sw 22 is on. At this time, the signal current I_(data),set by the reference constant-current source 21, flows in a directiontoward the power line 25. At this time, the current I_(data) is branchedinto I₁ and I₂ to flow. Incidentally, the current I_(data) satisfiesI_(data)=I₁+I₂.

In an instant a current begins to flow from the referenceconstant-current source 21, no charge is held on the capacitance element24. Consequently, the transistor 23 is off. Accordingly, this results inI₂=0 and I_(data)=I₁.

Then, charge gradually builds up on the capacitance element 24, to begincausing a potential difference at between the both electrodes of thecapacitance elements 24. When the potential difference at between theboth electrodes becomes a threshold voltage of the transistor 23, thetransistor 23 turns on to give I₂>0. Because of I_(data)=I₁+I₂ as in theforegoing, the current remains flowing despite I₁ gradually decreases(point A, FIGS. 3C and 3D).

The potential difference at between the both electrodes of thecapacitance element 24 provides a gate-to-source voltage for thetransistor 23. Consequently, charge storage is continued to thecapacitance element 24 until the transistor 23 reaches a voltage (VGS)capable of flowing a signal current as a desired current. Completing thecharge storage (point B, FIGS. 3C and 3D), the current I₁ ceases toflow. Furthermore, because the transistor 23 is on, I_(data)=I₂ results.

Subsequently, the sw 22 is turned off as shown in FIG. 3B. Because theVGS written in the foregoing operation is held on the capacitanceelement 24, the transistor 23 is on. Furthermore, a current equal to thesignal current I_(data) flows through the drain region of the transistor23. At this time, by allowing the transistor 23 to operate in asaturation region, even if there is a change in the source-to-drainvoltage of the transistor 23, the drain current of the transistor 23 canflow without a change in the value thereof.

As in the foregoing, in order to cause a current same as a signalcurrent set in the reference constant-current source to flow to aparticular transistor, a gate-to-source voltage may be set of thattransistor. In the invention, setting is possible by holding thegate-to-source voltage of the transistor due to a capacitance elementconnected to that transistor. By utilizing the voltage held on thecapacitance element, it is possible to suppress against the affection oftransistor characteristic variation.

The method of utilizing a voltage held on a capacitance element can usethe method shown in the below. The voltage held on a capacitance elementis held as it is, and a signal voltage (e.g. video signal voltage) isinputted to one terminal of the capacitance element. If doing so, thegate electrode of the transistor is inputted by a voltage that thevoltage held on the capacitance element is added to the signal voltage.As a result, the gate electrode of the transistor is inputted by a valuehaving the voltage held on the capacitance element added to the signalvoltage. Namely, in the invention, even where characteristic variationoccurs between transistors, the transistor a signal voltage is to beinputted is inputted by a value that a voltage held on each capacitanceelement each transistor is connected is added to the signal voltage.Accordingly, an electric circuit can be provided that is suppressedagainst the affection of the characteristic variation betweentransistors.

Note that the mechanism for adding a voltage held on a capacitanceelement to a signal voltage is to be explained by the chargeconservation law. The charge conservation law represents a fact that thearithmetic sum in amount of positive electricity and negativeelectricity is constant in total electricity amount.

The invention can use a transistor using any material or transistorprocessed by any means or manufacture method or transistor in any type.For example, a thin-film transistor (TFT) may be used. The TFT may use asemiconductor layer formed of any of amorphous, poly-crystal and singlecrystal ones. As another transistor, the transistor may be the onefabricated on a single-crystal substrate or transistor made on an SOIsubstrate. Besides, the transistor may be formed of an organic materialor carbon nano tube. Furthermore, MOS transistors or bipolar transistorsare also applicable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram explaining the operation of a source-followercircuit of the present invention;

FIGS. 2A and 2B are diagrams explaining the operation of thesource-follower circuit of the invention;

FIGS. 3A to 3D are diagrams explaining the configuration and operationof an electric circuit of the invention;

FIGS. 4A to 4H are figures of electronic apparatus to which theinvention is to be applied;

FIGS. 5A to 5C are diagrams explaining the operation of the sourcefollower circuit;

FIGS. 6A and 6B are diagrams explaining the operation of the sourcefollower circuit;

FIGS. 7A and 7B are diagrams showing a source-follower circuit of theinvention;

FIGS. 8A and 8B are diagrams showing a source-follower circuit of theinvention;

FIG. 9 is a diagram showing a source-follower circuit of the invention;

FIG. 10 is a diagram showing a differential amplifier circuit of theinvention;

FIG. 11 is a diagram showing a differential amplifier circuit of theinvention;

FIGS. 12A and 12B are diagrams showing an operational amplifier of theinvention;

FIGS. 13A and 13B are diagrams showing an operational amplifier of theinvention;

FIGS. 14A to 14C are diagrams showing a semiconductor device of theinvention;

FIG. 15 is a diagram showing a pixel and bias circuit of thesemiconductor device of the invention;

FIGS. 16A and 16B are diagrams explaining the configuration of theelectric circuit of the invention;

FIG. 17 is a diagram of a signal-line drive circuit of the invention;

FIG. 18 is a diagram of the signal-line drive circuit of the invention;

FIG. 19 is a diagram explaining the operation of the signal-line drivecircuit of the invention;

FIGS. 20A and 20B are diagrams showing a reference constant-currentsource;

FIGS. 21A to 21F are diagrams showing a reference constant-currentsource;

FIGS. 22A to 22E are diagrams showing a reference constant-currentsource;

FIGS. 23A and 23B are diagrams showing a reference constant-currentsource;

FIG. 24 is a diagram showing a source-follower circuit of the invention;

FIGS. 25A and 25B are diagrams showing a source-follower circuit of theinvention;

FIGS. 26A and 26B are diagrams showing a source-follower circuit of theinvention;

FIG. 27 is a diagram showing a source-follower circuit of the invention;

FIGS. 28A and 28B are diagrams showing a source-follower circuit of theinvention;

FIG. 29 is a diagram showing a source-follower circuit of the invention;

FIG. 30 is a diagram showing a differential amplifier circuit of theinvention;

FIG. 31 is a diagram showing a differential amplifier circuit of theinvention;

FIG. 32 is a diagram showing a differential amplifier circuit of theinvention;

FIG. 33 is a diagram showing a differential amplifier circuit of theinvention;

FIG. 34 is a diagram showing a differential amplifier circuit of theinvention;

FIG. 35 is a diagram showing a differential amplifier circuit of theinvention;

FIG. 36 is a diagram showing a differential amplifier circuit of theinvention;

FIG. 37 is a diagram showing a differential amplifier circuit of theinvention;

FIGS. 38A and 38B are diagrams showing an operational amplifier of theinvention;

FIGS. 39A and 39B are diagrams showing an operational amplifier of theinvention;

FIG. 40 is a diagram of a signal-line drive circuit of the invention;

FIG. 41 is a diagram of a signal-line drive circuit of the invention;and

FIG. 42 is a diagram explaining the operation of the signal-line drivecircuit of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Embodiment 1]

This embodiment shows a source-follower circuit as an example of anelectric circuit of the present invention, the configuration andoperation of which will be explained using FIGS. 1, 2A and 2B.

First explained is a configuration of the source-follower circuit of theinvention, by using FIGS. 1, 2A and 2B.

In FIGS. 1, 2A and 2B, 111 is an n-channel amplifier transistor, and 112is an n-channel bias transistor. 113 and 114 are capacitance elements.Meanwhile, 115-118, 120, 127, 128 are elements having switchingfunctions, which preferably use semiconductor elements, such as analogswitches, configured by transistors. In this case, the semiconductordevices are merely switches and hence not especially limited in theirpolarities.

126 is a reference constant-current source having a capability to flow aconstant current. The reference constant-current source 126 isconfigured by a semiconductor element of a transistor or the like. Inthe present specification, a reference constant-current source 126configured by transistor will be explained in its one example inEmbodiment 6. This can be made reference to conveniently.

123-125 are power lines, i.e. the power line 123 is applied with a powersource potential V_(dd1) while the power line 124 is with a groundpotential V_(ss). The power line 125 is applied with a power sourcepotential V_(dd2). The power source potential V_(dd1) applied to thepower line 123 and the power source potential V_(dd2) applied to thepower line 125 may be the same or different in value. However, the powersource potential V_(dd2) applied to the power line 125 is required to beset at a value that the reference constant-current source 126 is allowedto normally operate as a constant-current source. For example, where thereference constant-current source 126 utilizes a saturation region of atransistor to configure the current source, there is a need to set at avalue in a range the transistor is allowed to operate in the saturationregion.

Although this embodiment shows the case the amplifier transistor 111 andbias transistor 112 are of the n-channel type, the invention is notlimited to this, i.e. the both transistors may be of a p-channel type.Otherwise, the both transistors may be different in polarity toconfigure a push-pull circuit. It is noted that, where a push-pullcircuit is configured, the both transistors function as amplifiertransistors as shown in FIG. 24. Hence, signal input is to the bothtransistors.

The amplifier transistor 111 has a drain region connected to the powerline 123 through the switch 127, and a source region connected to theswitches 117, 118 and to a drain region of the transistor 112. Theamplifier transistor 111 has a gate electrode connected to one terminalof the capacitance element 113. The other terminal of the capacitanceelement 113 is connected to the source region of the transistor 111through the switch 117. The capacitance element 113 has a role to hold agate-to-source voltage of the amplifier transistor 111. Note that,hereinafter, the amplifier transistor 111 is denoted as the transistor111.

The bias transistor 112 has a source region connected to the power line124 and a drain region connected to the switches 117, 118 and 120. Thebias transistor 112 has a gate electrode connected to one terminal ofthe capacitance element 114. The other terminal of the capacitanceelement 114 is connected to the source region of the bias transistor112. The capacitance element 114 has a role to hold a gate-to-sourcevoltage of the bias transistor 112. Note that, hereinafter, the biastransistor 112 is denoted as the transistor 112.

The switches 115-118, 120, 127, 128 are controlled of conduction andnon-conduction (on and off) depending upon an input signal. However, inFIGS. 1, 2A and 2B, the signal lines or the like for input signals tothe switches 115-118, 120, 127, 128 are omittedly shown in order tosimplify explanation.

In the source-follower circuit of FIGS. 1, 2A and 2B, the switch 116 hasone terminal serving as an input terminal Through the input terminal, aninput potential V_(in) (signal voltage) is inputted to one terminal ofthe capacitance element 113. Meanwhile, the switch 118 has one terminalserving as an output terminal. The potential on the source region of thetransistor 111 provides an output potential V_(out).

Explanation is now made on the operation of the source-follower circuitof FIGS. 1, 2A and 2B.

In FIG. 1, the switches 115, 117, 120 and 128 are turned on. The otherswitches than the above are off. In this state, a signal currentI_(data) set in the reference constant-current source 126 flows towardthe power line 124 through the capacitance elements 113, 114.

In an instant a current begins to flow from the referenceconstant-current source 126, no charge is being held on the capacitanceelements 113, 114. Consequently, the transistors 111, 112 are off. Thecurrent flows in a direction from the reference constant-current source126 toward the power line 124 through the switches 128, 115, 117 andfurther through the switch 120.

Charge are gradually built up on the capacitance elements 113, 114 tobegin causing a potential difference at between the both electrodes ofthe capacitance 113, 114. When the potential difference at between theboth electrodes of the capacitance element 113 reaches a thresholdvoltage V_(th1) of the transistor 111, the transistor 111 turns on.Similarly, when the potential difference at between the both electrodesof the capacitance element 114 reaches a threshold voltage V_(th2) ofthe transistor 112, the transistor 112 turns on.

Then, charge storage is continued onto the capacitance element 113 sothat the gate-to-source voltage of the transistor 111 can flow apredetermined signal current I_(data). Also, charge storage is continuedonto the capacitance element 114 so that the gate-to-source voltage ofthe transistor 112 can flow a predetermined signal current I_(data).

As shown in FIG. 2A, when the capacitance elements 113, 114 complete theelectric-charge storage into a steady state, the switches 115, 117, 120are turned from on to off while the other switches are maintained in thestate of FIG. 1. At this time, the signal current I_(data) set by thereference constant-current source 126 flow through the drain to sourceregion of the transistor 111 and further the drain to source region ofthe transistor 112. Incidentally, it is assumed that the potentialdifference is V_(a) at between the both electrodes of the capacitanceelement 113 while the potential difference is V_(c) at between the bothelectrodes of the capacitance element 114.

Subsequently, the switches 116, 118, 127 are turned on, as shown in FIG.2B. The other switches than the above are all turned off. At this time,an input potential V_(in) is inputted from the input terminal to oneterminal of the capacitance element 113 through the switch 116. By thecharge conservation law, the gate electrode of the transistor 111 isapplied by a value (V_(a)+V_(in)) that the input potential V_(in) isadded to the gate-to-source voltage V_(a) of the transistor 111.

The output potential V_(out) is at a potential on the source region ofthe transistor 111. Namely, this corresponds to a value that thegate-to-source voltage V_(gs) (=V_(a)) is subtracted from the gatepotential (V_(in)+V_(a)) of the transistor 111.

Incidentally, after the switch 128 is turned off and the switch 127turned on, the signal current I_(data) also flows through the transistor111. This is because that the gate-to-source voltage V_(gs) (=V_(c)) ofthe transistor 112 is added with a voltage required to flow the signalcurrent I_(data). Accordingly, the gate-to-source voltage V_(gs) of thetransistor 111 is also added with a voltage required to flow the signalcurrent I_(data). The required voltage is the voltage denoted by V_(a).Consequently, it can be seen that the gate-to-source voltage V_(gs) ofthe transistor 111 has the same value as V_(a). Summarizing, thefollowing Equation (4) is held.V _(out)=(V _(in) +V _(a))−V _(a) =V _(in)  (4)

As shown in Equation (4), the output potential V_(out) is the same invalue as the input potential V_(in), and not dependent upon thetransistor characteristic. Consequently, should characteristic variationoccur in the transistor 111 and transistor 112, it can be suppressedfrom having an effect upon the output potential V_(out).

Although the electric circuit of FIGS. 1, 2A and 2B is a source-followercircuit, there is no provision of an input terminal for inputting a biaspotential. This is because, at between the gate-to-source of thetransistor 112, predetermined charge is already held on the capacitanceelement 114 to flow the signal current I_(data) set by the referenceconstant-current source 126.

Because the invention can suppress against the affection ofcharacteristic variation of the transistors 111 and 112, there is noneed to design the transistors 111 and 112 with the same value of gatelength (L) and gate width (W). There is no problem if variation occurs.

In this specification, the operation to hold predetermined charge on acapacitance element is referred to as a setting operation. In thisembodiment, the operation in FIGS. 1 and 2A corresponds to a settingoperation. Also, the operation to input an input potential V_(in) andtake out an output potential V_(out) is referred to as an outputoperation. In this embodiment, the operation of FIG. 2B corresponds toan output operation.

Incidentally, although the electric circuit of FIGS. 1, 2A and 2B has aconnection in the order of the power line 125, the referenceconstant-current source 126 and the switch 128, the invention is notlimited to this. For example, the connection may be in the order of thepower line 125, the switch 128 and the reference constant-current source126 by reversing the reference constant-current source 126 and theswitch 128.

Meanwhile, the reference constant-current source 126 may be arranged asshown in FIG. 7A or 7B. Explanation will be made on the electric circuitconfigurations shown in FIGS. 7A and 7B. The electric circuit of FIG. 7Aor 7B has the same circuit elements as those of the electric circuit ofFIGS. 1, 2A and 2B, excepting that the power line 125 is not provided.The power line 123 is applied with a power-source potential V_(dd) whilethe power line 124 is with a ground potential V_(ss). The operation ofthe source-follower circuit of FIG. 7A or 7B is similar to the operationof the source-follower circuit of FIGS. 1, 2A and 2B, and henceomittedly explained in this embodiment.

In FIG. 7A, the switch 127 is arranged between the drain region of thetransistor 112 and the power line 124. The switch 128 is arranged,parallel with the switch 127, between the drain region of the transistor112 and the power line 124. Finally, the reference constant-currentsource 126 is arranged between the drain region of the transistor 112and the switch 128 or between the switch 128 and the power line 124.FIG. 7A shows a case of the arrangement at between the drain region ofthe transistor 112 and the switch 128.

In FIG. 7A, the switches 127 and 128 are both connected to the groundpotential V_(ss). However, the invention is not limited to this. Thesemay be connected to different power lines in a manner such that, in FIG.1, the switch 127 is connected to the power-source potential V_(dd1) andthe switch 128 is to the power-source potential V_(dd2). For example,the switch 127 may be connected to the ground potential V_(ss) as inFIG. 7A while the switch 128 be connected to a newly-arranged groundpotential V_(ss2). The ground potential V_(ss) and the ground potentialV_(ss2) may be at the same value or different values.

In FIG. 7B, the switch 127 is arranged at between the source region ofthe transistor 111 and the drain region of the transistor 112. Theswitch 128 is arranged in parallel with the switch 127. Finally, thereference constant-current source 126 is arranged between the sourceregion of the transistor 111 and the switch 128 or between the switch128 and the drain region of the transistor 112. FIG. 7B shows a case ofan arrangement at between the source region of the transistor 111 andthe switch 128.

Although, in FIG. 7B, the switch 118 is connected to the source regionof the transistor 111 and to the drain region of the transistor 112through the switch 127, the invention is not limited to this. The switch118 may be connected to the drain region of the transistor 112 and tothe source region of the transistor 111 through the switch 127.

However, it is preferred that the switch 118 is connected to the sourceregion of the transistor 111 and to the drain region of the transistor112 through the switch 127. This is because that, in the case the switch118 is connected to the drain region of the transistor 112 and to thesource region of the transistor 111 through the switch 127, if there isan on-resistance through the switch 127, it has an effect upon theoutput potential V_(out) to lower the output potential V_(out).

Meanwhile, FIG. 8A shows a source-follower circuit that, in the electriccircuit of FIGS. 1, 2A and 2B, a switch 119 is arranged between thedrain region of the transistor 111 and the power line 124 withoutarranging a transistor 112, capacitance element 114 and switch 120. Theoperation of the source-follower circuit of FIG. 8A is similar to theforegoing operation of FIGS. 1, 2A and 2B excepting that the switch 119is on during a setting operation and off during an output operation,hence omittedly explained in this embodiment.

In FIG. 8A, the switches 127, 128 and the current source 126 areconnected to the power-source potential V_(dd), similarly to FIG. 1.However, the switches 127, 128 and the current source 126 may beconnected to another element, such as the ground potential V_(ss), asshown in FIGS. 7A and 7B. FIG. 25A shows, as an example, a case that theswitches 127, 128 and the current source 126 are connected to the groundpotential V_(ss).

Herein, FIG. 25A shows a source-follower circuit in a case not providedwith the transistor 112. However, the transistor 112, in its nature, isa circuit to be operated as a current source for providing a bias in thesource-follower circuit. Accordingly, the current source 126 in FIG. 25Amay be operated as a current source to provide a bias in place of thetransistor 112. Namely, the current source 126 may be used as a currentsource to set the transistor 111 during a setting operation and as acurrent source to supply a bias in the source-follower circuit during anoutput operation, instead of being used during a setting operation butnot used during an output operation. In this case, there is no need forswitching at between setting and output operations, thus eliminating thenecessity of the switches 127, 128. The circuit diagram, in this case,is shown in FIG. 26A.

Meanwhile, there is shown, in FIG. 27, a circuit diagram that thecurrent source of FIG. 26A is realized by a transistor. Next, theoperation will be shown.

In FIG. 27, the switches 115, 117 are turned on. The other switches thanthe above are turned off. In this state, the signal current I_(data) setin the transistor 112 flows in a direction toward the power line 124through the capacitance element 113. The magnitude of the signal currentI_(data) is determined by a bias voltage Vb applied to the gate of thetransistor 112 and a characteristic of the transistor 112. Accordingly,if there should be a plurality of circuits of FIG. 27, there is apossibility of variation in the characteristic of the transistor 112 inthe plurality of circuits. In such a case, even if the same voltage Vbis applied to the gate of each transistor 112, the magnitude of signalcurrent I_(data) is different between the circuits.

At the instant of flowing current from the transistor 112, no charge isstored on the capacitance element 113. Consequently, the transistor 111is off. The current flows in a direction toward the power line 124 fromthe transistor 112 through the switches 115, 117.

Charge is gradually built up on the capacitance element 113, and apotential difference begins to occur at between the both electrodes ofthe capacitance element 113. When the potential difference at betweenthe electrodes of the capacitance element 113 becomes a thresholdvoltage V_(th1) of the transistor 111, the transistor 111 turns on.

Then, charge storage is continued to the capacitance element 113 suchthat the gate-to-source voltage of the transistor 111 becomes a voltagecapable of flowing a predetermined signal current I_(data).

As shown in FIG. 28A, when the capacitance element 113 is completed ofcharge storage into a steady state, the switches 115, 117 are turnedfrom on to off while the other switches than these are kept in the stateof FIG. 27. At this time, the signal current I_(data) flowed from thetransistor 112 flows from the drain to source region of the transistor111. Incidentally, it is assumed that the potential difference atbetween the both electrodes of the capacitance element 113 is V_(a).

If the circuit of FIG. 27 should exist in plurality, there is apossibility that the transistors 111, 112 have characteristic variationbetween the circuits. In this case, the magnitude of the signal currentI_(data) differs from circuit to circuit. Similarly, the potentialdifference V_(a) at between the both electrodes of the capacitanceelement 113 is also differs from circuit to circuit.

Subsequently, as shown in FIG. 28B, the switches 116, 118 are turned on.The other switches than the above are all turned off. At this time, aninput potential V_(in) is inputted from the input terminal through theswitch 116 to one terminal of the capacitance element 113. By the chargeconservation law, the gate electrode of the transistor 111 is applied bya value (V_(a)+V_(in)) that the input potential V_(in) is added to thegate-to-source voltage V_(a).

The output potential V_(out) is at a potential on the source region ofthe transistor 111. Namely, the output potential V_(out) corresponds toa value that the gate-to-source voltage V_(gs)(=V_(a)) is subtractedfrom the gate potential (V_(in)+V_(a)) of the transistor 111.

The signal current I_(data) continues to flow through the transistor111. This is because the gate voltage V_(b) of the transistor 112remains at the same value. Accordingly, the gate-to-source voltageV_(gs) of the transistor 111 is also being applied by a voltage requiredfor the transistor 111 to flow the signal current I_(data). The requiredvoltage is a voltage denoted by V_(a). Accordingly, it can be seen thatthe gate-to-source voltage V_(gs) of the transistor 111 is in the samevalue as V_(a). Summarizing the above, Equation (4) is also held herein.

As shown in Equation (4), the output potential V_(out) is in the samevalue as the input potential V_(in), and not dependent upon thetransistor characteristic. Consequently, if there is characteristicvariation on the transistors 111 and 112, it is possible to suppressagainst an effect of characteristic variation upon the output potentialV_(out).

If the circuit of FIG. 27 should exist in plurality, there is apossibility that the characteristic of the transistor 112 or 111 variesbetween the circuits. In such a case, the magnitude of signal currentI_(data) and the electrode-to-electrode potential difference V_(a) oncapacitance element 113 are different between the circuits. However, asshown in Equation (4), the output potential V_(out) is in the same valueas the input potential V_(in), and not dependent upon the magnitude ofsignal current I_(data) and the electrode-to-electrode potentialdifference V_(a) on the capacitance element 113. Namely, where thecircuit of FIG. 27 is exists in plurality, even if the characteristic ofthe transistor 112 or 111 varies between the circuits, the affectionthereof can be relaxed.

Because the invention can suppress the affection of characteristicvariation of the transistors 111 and 112, there is no need to design thetransistors 111 and 112 with the same value of gate length (L) and gatewidth (W). There is no problem if variation occurs.

Next, comparison is made between a case to supply a current from anoutside of the source-follower circuit as shown in FIG. 1 and a case tocarry out also a setting operation by using a bias current source to thesource-follower circuit as shown in FIGS. 26A and 26B.

Considering at first circuit configuration, the arrangement of FIGS. 26Aand 26B is simpler and hence advantageous. Particularly, in the case ofarranging a plurality of source-follower circuits, it is moreadvantageous. However, the arrangement of FIGS. 26A and 26B where thereare a plurality of source-follower circuits, the current flowing througheach circuit possibly differs in value due to the variation in thecurrent source or the like. As a result, the input voltage and outputvoltage, when a steady state is reached, are equal on everysource-follower circuit. However, there possibly occurs a case thattransient characteristic is different between the source-followercircuits.

On the other hand, in the case of FIG. 1, circuit configuration is morecomplicated because of the necessity to supply a current from an outsideof the source-follower circuit. Particularly, where a plurality ofsource-follower circuits are arranged, the circuit configuration thereofis further complicated. In the case that the current source 126 of FIG.1 should be arranged one and the source-follower circuit is arranged inplurality, it is impossible to carry out setting operationssimultaneously on all the source-follower circuits. Consequently,operation timing is complicated. Otherwise, in the case that the currentsource 126 of FIG. 1 is provided in the same number as thesource-follower circuits, the current sources 126 desirably have novariations.

However, in the case there are a plurality of source-follower circuits,even if the source-follower circuits have variations in characteristic,there occurs no variation in the value of the current flowing throughthe source-follower circuit. This is because the current value isdetermined by the current source 126 provided outside thesource-follower circuit. Therefore, there encounters no variation intransient characteristic, besides steady state characteristic, betweenthe source-follower circuits.

In this manner, in the invention, even if there is a characteristicvariation occurring between transistors, the transistor, to be inputtedby a signal voltage of an input potential V_(in) or the like, isinputted, without exception, by a value that the gate-to-source voltageof the transistor is added with the signal voltage. Accordingly, it ispossible to provide an electric circuit suppressed against the affectionof characteristic variation between transistors.

[Embodiment 2]

The source-follower circuit of FIGS. 1, 2A and 2B showed theconfiguration with the n-channel amplifier transistor 111 and then-channel bias transistor 112. Next, this embodiment shows, in FIG. 9, asource-follower circuit configured with a p-channel amplifier circuit132 and a p-channel bias transistor 131, the configuration of which willbe explained. Note that the operation of the source-follower circuit ofFIG. 9 is similarly to the operation of the source-follower circuit ofFIGS. 1, 2A and 2B explained in Embodiment 1 and hence omittedlyexplained in this embodiment.

In FIG. 9, 131 is a p-channel bias transistor while 132 is a p-channelamplifier transistor. 133 and 134 are capacitance elements. Meanwhile,135, 136, 138-142 are elements having switching functions, whichpreferably use semiconductor elements, such as analog switches,configured by transistors.

146 is a reference constant-current source having a capability to flow aconstant current. The reference constant-current source 146 isconfigured by a semiconductor element, such as a transistor. In thepresent specification, the reference constant-current source 146configured by a transistor will be explained in its one example inEmbodiment 6. This can be made reference to conveniently.

143-145 are power lines. The power line 143 is applied with apower-source potential V_(dd1) while the power line 144 is applied witha ground potential V_(ss). The power line 145 is applied with a powersource potential V_(dd2). Incidentally, the power source potentialV_(dd1) applied to the power line 143 and the power source voltageV_(dd2) applied to the power line 145 may be the same or different invalue. However, the power source potential V_(dd2) applied to the powerline 145 is required to be set at a value that the referenceconstant-current source 146 is allowed to normally operate as aconstant-current source. For example, where the referenceconstant-current source 146 utilizes a saturation region of a transistorto configure the current source, there is a need to set at a value thatthe transistor is allowed to operate in the saturation region.

Although this embodiment shows the case the amplifier transistor 132 andbias transistor 131 are of the p-channel type, the invention is notlimited to this, i.e. the both transistors may be different in polarityto configure a push-pull circuit.

The bias transistor 131 has a source region connected to the power line143 through the switch 136 and a drain region connected to the switches135, 138, 142. The bias transistor 131 has a gate electrode connected toone terminal of the capacitance element 133. The other terminal of thecapacitance element 133 is connected to the power line 143 through theswitch 136. The capacitance element 133 has a role to hold agate-to-source voltage of the bias transistor 131.

The amplifier transistor 132 has a drain region connected to the powerline 144 and a source region connected to switches 138, 142. Theamplifier transistor 132 has a gate electrode connected to one terminalof the capacitance element 134. The other terminal of the capacitanceelement 134 is connected to the source region of the amplifiertransistor 132 through the switch 142. The capacitance element 134 has arole to hold a gate-to-source voltage of the amplifier transistor 132.

The switches 135, 136, 138-142 are controlled of conduction andnon-conduction (on and off) according to an input signal. However, inFIG. 9, the signal lines or the like for inputting signals to theswitches 135, 136, 138-142 are omittedly shown in order to simplifyexplanation.

In the source-follower circuit of FIG. 9, the switch 141 has oneterminal serving as an input terminal. Through the input terminal, aninput potential V_(in) (signal voltage) is inputted to one terminal ofthe capacitance element 134. Meanwhile, the switch 138 has one terminalserving as an output terminal. The potential on the source region of theamplifier transistor 132 provides an output potential V_(out).

Although the electric circuit shown in FIGS. 1, 2A and 2B is asource-follower circuit, there is no provision of an input terminal forinputting a bias potential. This is because predetermined charge isalready held on the capacitance element 114 to flow the signal currentI_(data) set by the reference constant-current source 126 through thegate to source of the transistor 131.

Because the invention can suppress against the affection ofcharacteristic variation of the bias transistor 131 and amplifiertransistor 132, there is no need to design the bias transistor 131 andamplifier transistor 132 with the same value of gate length (L) and gatewidth (W). There is no problem if variation occurs.

Although the connection in FIG. 9 is in the order of the power line 145,the reference constant-current source 146 and the switch 139, theinvention is not limited to this. The connection may be in the order ofthe power line 145, the switch 139 and the reference constant-currentsource 146 by reversing the reference constant-current source 146 andthe switch 139.

Meanwhile, by referring to the foregoing Embodiment 1 and FIGS. 7A and7B, the reference constant-current source 146 may be arranged at betweenthe switch 140 and the power line 144. Furthermore, the referenceconstant-current source 146 may be arranged at between the switch 138and the switch 142.

FIG. 8B shows a source-follower circuit in a case not provided with thebias transistor 131, the capacitance element 133 and the switch 135. Theoperation of the source-follower circuit of FIG. 8B is similar to theforegoing operation of FIGS. 1, 2A and 2B in Embodiment 1, and henceomittedly explained in this embodiment.

In FIG. 8B, the switch 136, the switch 139 and the current source 146are connected to the power-source potential V_(dd), similarly to FIG. 1.However, the switch 136, the switch 139 and the current source 146 maybe connected to another power line or element, such as the groundpotential V_(ss), as in FIG. 7A or 7B. FIG. 25B shows, as an example, acase that the switch 136, the switch 139 and the current source 146 areconnected to the ground potential V_(ss).

Herein, FIG. 8B shows the source-follower circuit in the case thetransistor 131 is not provided. However, the transistor 131, in itsnature, is a circuit to be operated as a current source for providing abias in the source-follower circuit. Accordingly, the current source 146in FIG. 8B may be operated as a current source for providing a bias inplace of the transistor 131. Namely, the current source 146 may be usedas a current source to set the transistor 132 during a setting operationand as a current source to supply a bias in the source-follower circuitduring an output operation, instead of being used during a settingoperation but not used during an output operation. In such a case, thereis no need for switching between a setting operation and an outputoperation, thus eliminating the necessity of the switches 136, 139. Thecircuit diagram, in this case, is shown in FIG. 26B.

In FIG. 29 is shown a circuit diagram in a case that the current source146 of FIG. 26B is realized by a transistor. The operation of thesource-follower circuit of FIG. 29 is similar to the foregoing operationof FIG. 27 or 28 in Embodiment 1, and hence omittedly explained in thisembodiment.

This embodiment can be desirably combined with Embodiment 1.

[Embodiment 3]

The foregoing Embodiments 1, 2A and 2B explained the source-followercircuits to which the invention is applied. However, the invention isapplicable to various circuits, including a differential amplifiercircuit, a sense amplifier and an operation amplifier. This embodimentexplains an operating circuit the invention is applied, by using FIGS.10 to 13.

First explained is a differential amplifier circuit the invention isapplied, by using FIG. 10. FIG. 10 corresponds to a case arranged with areference constant-current source 268 besides the native circuitsimilarly to FIG. 1. The differential amplifier circuit carries out anoperation on a difference between an input potential V_(in1) and aninput potential V_(in2), to output an output potential V_(out).

In the differential amplifier circuit of FIG. 10, 272, 273 are p-channeltransistors while 274, 275 and 286 are n-channel transistors. 276, 277and 278 are capacitance elements. Meanwhile, switches 265, 266, 278-284,288, 502 and 503 are elements having switching functions, whichpreferably use semiconductor elements, such as transistors. Thesemiconductor elements are not especially limited in polarity.

268 is a reference constant-current source having a capability to flow aconstant current. The reference constant-current source 268 isconfigured by a semiconductor element, such as a transistor. In thepresent specification, the reference constant-current source 268configured by a transistor will be explained in its one example inEmbodiment 6. This can be made reference to conveniently.

267, 271 and 291 are power lines. The power line 271 is applied with apower source potential V_(dd1) while the power line 291 is with a groundpotential V_(ss). The power line 267 is applied with a power sourcepotential V_(dd2). The power source potential V_(dd1) applied to thepower line 271 and the power source potential V_(dd2) applied to thepower line 267 may be the same or different in value. However, the powersource potential V_(dd2) applied to the power line 267 is required to beset at a value that the reference constant-current source 268 is allowedto normally operate as a constant-current source. For example, where thereference constant-current source 268 utilizes a saturation region of atransistor to configure the current source, there is a need to set at avalue that the transistor is allowed to operate in the saturationregion.

In the differential amplifier circuit of FIG. 10, the switch 281 has oneterminal serving as an input terminal. An input potential V_(in1) isinputted to one terminal of the capacitance element 276. Meanwhile, theswitch 284 has one terminal serving as an input terminal. An inputpotential V_(in2) is inputted to one terminal of the capacitance element277. Also, the transistor 275 has a drain region made as an outputterminal so that the potential on the drain region of the transistor 275provides an output potential V_(out).

The transistor 272 has a drain region connected to the power line 271and a source region connected to a drain region of the transistor 274through the switch 502. The transistor 273 has a drain region connectedto the power line 271 and a source region connected to a drain region ofthe transistor 275 through the switch 503. The gate electrode of thetransistor 272 and the date electrode of the transistor 273 areconnected together. Incidentally, resistances may be arranged in placeof the transistors 272 and 273. This is because, in the differentialamplifier circuit as in FIG. 10, 272, 273 are parts called active loadsto be operated as resistances. Consequently, the parts of active loadsin FIG. 10 may be configured by usual resistance elements as in FIG. 30.

The transistor 274 has a drain region connected to the power line 271through the switch 502 and transistor 272, and a source region connectedto one terminal of the capacitance element 276 through the switch 282.The transistor 274 has a gate electrode connected to the other terminalof the capacitance element 276. The capacitance element 276 plays a roleto hold a gate-to-source voltage of the transistor 274 when carrying outa setting operation.

The transistor 275 has a drain region connected to the power line 272through the switch 503 and transistor 273, and a source region connectedto one terminal of the capacitance element 277 through the switch 283.The transistor 275 has a gate electrode connected to the other terminalof the capacitance element 277. The capacitance element 277 plays a roleto hold a gate-to-source voltage of the transistor 275 when carrying outa setting operation.

The transistor 286 has a drain region connected to the source region ofthe transistor 274 and to the source region of the transistor 275. Thetransistor 286 has a source region connected to one terminal of thecapacitance element 287. The gate electrode of the transistor 286 isconnected to the other terminal of the capacitance element 287. Thecapacitance element 287 plays a role to hold a gate-to-source voltage ofthe transistor 286.

Predetermined charge is held onto the capacitance elements 276, 277 and287 by the use of the reference constant-current source 268. However,predetermined charge cannot be held, at one time, onto the threecapacitance elements 276, 277 and 287. For this reason, it is carriedout under control such that one of the switches 265 and 266 is turnedon. For example, when the switch 265 is turned on, the switch 266 isturned off. Then, predetermined charge is held onto the capacitanceelements 277, 287. Similarly, the switch 265 is turned off and theswitch 266 is turned on. Then, predetermined charge is held onto thecapacitance elements 276, 287.

Incidentally, the explanation of the operation during holdingpredetermined charge on the capacitance elements 276, 277 and 287 byusing the reference constant-current source 268 is similar to that ofEmbodiment 1, and hence omittedly explained in this embodiment.

After completing the holding of predetermined charge on the capacitanceelement 276, an input potential V_(in1) is inputted to one terminal ofthe capacitance element 276. Also, after completing the holding ofpredetermined charge on the capacitance element 277, an input potentialV_(in2) is inputted to one terminal of the capacitance element 277 tocarry out an output operation. The operation in this case is similar tothat of Embodiment 1 and hence omittedly explained in this embodiment.

Next, explanation is made on a differential amplifier circuit to whichis applied a circuit that a setting operation is carried out byutilizing a current source possessed by the native circuit as in FIGS.26 and 27, by using FIG. 31.

FIG. 10 used a current supplied, as a current during a settingoperation, from the current source 268. In FIG. 31, a setting operationis made by using the transistor 286. The transistor 286 operates as acurrent source, which determines a magnitude of current depending upon abias voltage V_(b) applied to the gate thereof.

Next described is the operation. First, as shown in FIG. 32, theswitches 504, 279, 282 are turned on while the other switches than thoseare turned off. Thereupon, a current flows toward the transistor 274,thus allowing a setting operation for the transistor 274. Next, as shownin FIG. 33, the switches 505, 280, 283 are turned on while the otherswitches than those are turned off. Thereupon, a current flows towardthe transistor 275, thus allowing a setting operation for the transistor275. This completes the setting operation. Consequently, as shown inFIG. 34, the switches 502, 503, 281, 284 are turned on while the otherswitches are turned off. Then, a normal operation is carried out.

Incidentally, it is possible to omit the switch 504 by turning on theswitch 502 during a setting operation of the transistor 274.

Meanwhile, the voltage to be applied to the gate of the transistor 286may be changed upon between a setting operation and a usual operation(output operation). Usually, the transistor 274 and the transistor 275,in many cases, have nearly the same amount of flowing current in thedifferential amplifier circuit. Accordingly, in the case of carrying outa setting operation, the setting operation is preferably done under thecondition approximate to that of a usual operation (output operation).This provides for higher accuracy. Accordingly, by adjusting the voltageto be applied to the gate of the transistor 286, it is preferred to flowa current in a half amount of that of a usual operation (outputoperation) during a setting operation.

Consequently, FIG. 35 shows a diagram in a case that a transistor 506 isarranged parallel with the transistor 286 as another method forobtaining the similar effect. The transistor 506 is desirably in a sizemade the same as the transistor 286. During a usual operation, the gateof the transistor 506 is applied by a voltage same as that of thetransistor 286. During a setting operation, a current is not allowed toflow through the transistor 506.

FIG. 36 shows a circuit diagram in a case the magnitude of a current ischanged upon between a usual operation and a setting operation by theswitch 507, as a circuit similar to that of FIG. 35. During a settingoperation, the switch 507 is turned off thereby reducing the amount ofcurrent to a half. During a usual operation, the switch 507 is turnedon. This can carry out a setting operation in a state approximate to astate of an actual operation, thus enhancing the effect of settingoperation.

Subsequently, explanation is made on a case that the transistorconstituting the differential amplifier circuit of FIG. 10 has anopposite conductivity type, by using FIG. 11.

In the differential amplifier of FIG. 11, reference numerals 272, 273are n-channel transistors while reference numerals 274, 275 and 286 arep-channel transistors. The switch 281 has one terminal as an inputterminal to input an input potential V_(in1) to one terminal of thecapacitance element 276. Also, the switch 284 has one terminal as aninput terminal to input an input potential V_(in2) to one terminal ofthe capacitance element 277. The potential on the source region of thetransistor 275 provides an output potential V_(out).

Incidentally, the differential amplifier circuit of FIG. 11 is similarin configuration and operation to the differential amplifier circuit ofFIG. 10 excepting that a power source potential V_(dd1) is applied tothe power line 291, a power source potential V_(dd2) is applied to thepower line 267 and a ground potential V_(ss) is applied to the powerline 271, and hence omittedly explained.

Incidentally, the differential amplifier circuit of FIG. 10 or 11 isdifferent in position arranging the reference constant-current source268. The invention is not limited in position arranging the referenceconstant-current source 268 but requires to satisfy the followingcondition.

It was mentioned in the foregoing that holding predetermined charge onthe capacitance elements 276, 277, 287 by using the referenceconstant-current source 268 is under control of the switches 265 and266. Namely, when the capacitance element 276 holds predetermined chargeunder control of the switches 265 and 266, there is a need not to flow acurrent to the capacitance element 277 and transistor 275. Similarly,when the capacitance element 277 holds predetermined charge, there is aneed not to flow a current to the capacitance element 276 and transistor274.

Namely, there is a need to arrange the reference constant-current source268 and the switches 265, 266, in order for the two capacitance elements276, 277 not to simultaneously hold predetermined charge. Also, there isa necessity to additionally arrange switches as required.

Considering the above, the arrangement position of the referenceconstant-current source 268 and switches 265, 266 is not limited to thepoint shown in FIG. 10 or 11. For example, in FIG. 11, the switch 265may be arranged between the power line 271 and the source region of thetransistor 272 while the switch 266 be between the power line 271 andthe source region of the transistor 273. Also, the switch 265 may bearranged between the drain region of the transistor 272 and the switch279 while the switch 266 be between the drain region of the transistor273 and the switch 280.

Next, FIG. 37 shows a case that the transistor configuring thedifferential amplifier circuit of FIG. 31 has an opposite conductivitytype. This is also similar in configuration and operation to thedifferential amplifier circuit of FIG. 31, and hence omittedly explainedherein.

Incidentally, the current value in the current-source part, in FIG. 37,can be similarly controlled by providing an arrangement as in FIG. 35 or36.

Although this embodiment showed the electric circuit of FIG. 10 or 11 asa differential amplifier circuit, the invention is not limited to this.It is possible to use it as another operating circuit, such as a senseamplifier, by properly changing the voltage to be inputted as an inputpotential V_(in1) and input potential V_(in2).

Next, an operational amplifier the invention is applied is explained, byusing FIGS. 12A, 12 B, 13A and 13B. FIG. 12A shows circuit symbolsconcerning an operational amplifier while FIG. 12B shows a circuitconfiguration of the operational amplifier.

It is noted that there are various operational-amplifier circuitconfigurations. Consequently, In FIGS. 12A and 12B, described is a casethat a differential amplifier circuit is combined with a source-followercircuit, as the simplest case. Hence, the operational-amplifier circuitconfiguration is not limited to FIGS. 12A and 12B.

The operational amplifier is defined in its characteristic by arelationship between an input potential V_(in1) and input potentialV_(in2) and an output potential V_(out). More specifically, theoperational amplifier has a function to multiply an amplification degreeA on a difference between an input potential V_(in1) and an inputpotential V_(in2), to output an output potential V_(out).

In the operational amplifier shown in FIG. 12B, a switch 281 has oneterminal as an input terminal to input an input potential V_(in1) to oneterminal of a capacitance element 276. A switch 284 has one terminal asan input terminal to input an input potential V_(in2) to one terminal ofa capacitance element 277. The potential on a source region of atransistor 292 provides an output potential V_(out).

In the circuit of FIG. 12B, the region surrounded by the dotted lineshown at 305 has the same configuration as the differential amplifiercircuit of FIG. 10. Furthermore, the region surrounded by the dottedline shown at 306 is the same as the source-follower circuit of FIGS. 1,2A and 2B. Hence, the detailed configuration of the operationalamplifier of FIG. 12B is omittedly explained.

In FIG. 12B, the current source 268 is commonly used by the differentialamplifier circuit 305 and the source-follower circuit 306.

Accordingly, FIGS. 38A and 38B show an operational amplifier in a casethat the region surrounded by the dotted line shown at 305 uses the sameconfiguration as the differential amplifier circuit of FIG. 31 while theregion surrounded by the dotted line shown at 306 uses the sameconfiguration as the source-follower circuit of FIG. 27.

Meanwhile, FIGS. 13A and 13B show an operational amplifier in a case thetransistor 299 is a p-channel transistor. Namely, this corresponds to acase using a push-pull circuit. FIG. 13B is the same in configuration asthe operation amplifier of FIG. 12B excepting that a capacitance element300, at one terminal, is connected to the drain region of the transistor275 through the switches 302, 278. Hence, this embodiment omittedlyexplains a detailed configuration.

FIGS. 39A and 39B show an operational amplifier in a case that theregion surrounded by the dotted line shown at 305 in FIG. 13B uses thesame configuration as the differential amplifier circuit of FIG. 31. InFIGS. 39A and 39B, the source-follower circuit part is made as apush-pull circuit and hence a bias current source does not exist.Consequently, the current of a current source of the differentialamplifier circuit is utilized as a current for use in a settingoperation of the source-follower circuit (push-pull circuit). Namely,the transistor 286 is connectable to the push-pull circuit.

Incidentally, this embodiment can be desirably combined with Embodiment1 or 2.

[Embodiment 4]

This embodiment explains the configuration and operation of asemiconductor device having a photoelectric element to which theinvention is applied, by using FIGS. 14A-14C and 15.

The semiconductor device shown in FIG. 14A has a pixel region 702 havinga plurality of pixels arranged in a matrix faun on a substrate 701.Around the pixel region 702, there are provided a signal-line drivecircuit 703 and first to fourth scanning-line drive circuits 704-707.Although the semiconductor device of FIG. 14A has the signal-line drivecircuit 703 and the first to fourth scanning-line drive circuits704-707, the invention is not limited to this, i.e. the signal-linedrive circuit and scanning-line drive circuits are arbitrarily arrangedin the number depending upon a pixel configuration. Also, signals areexternally supplied to the signal-line drive circuit 703 and first tofourth scanning-line drive circuits 704-707 through an FPC 708. However,the invention is not limited to this but the electric circuits otherthan the pixel region may be use an IC to externally supply signals.

First explained is a configuration of the first scanning-line drivecircuit 704 and second scanning-line drive circuit 705, by using FIG.14B. The third scanning-line drive circuit 706 and the fourthscanning-line drive circuit 707 conform to the diagram of FIG. 14B, andhence omittedly shown.

The first scanning-line drive circuit 704 has a shift register 709 and abuffer 710. The second scanning-line drive circuit 705 has a shiftregister 711 and a buffer 712. Briefly explain the operation, the shiftregister 709, 711 sequentially outputs sampling pulses according to aclock signal (G-CLK), start pulse (SP) and clock inversion signal(G-CLKb). Thereafter, the pulse amplified by the buffer 710, 712 isinputted to scanning lines and made in a selective state row by row.

Incidentally, configuration may be made such that a level shifter isarranged between the shift register 709 and the buffer 710 or betweenthe shift register 711 and the buffer 712. The arrangement of a levelshifter circuit can increase voltage amplitude.

Next explained is the configuration of the signal-line drive circuit703, by using FIG. 14C.

The signal-line drive circuit 703 has a signal-output-line drive circuit715, a sample hold circuit 716, a bias circuit 714 and an amplifiercircuit 717. The bias circuit 714, in a pair with an amplifiertransistor of each pixel, forms a source-follower circuit. The samplehold circuit 716 has a function to temporarily store a signal, make ananalog-digital conversion and reduce noise. The signal-output-line drivecircuit 715 has a signal output function to sequentially outputtemporarily stored signals. The amplifier circuit 717 has a circuit toamplify a signal outputted from the sample hold circuit 716 andsignal-output-line drive circuit 715. Incidentally, the amplifiercircuit 717 may not be arranged where no signal amplification isrequired.

Explanation is made on the configuration and operation of a circuit of apixel 713 arranged at i-th column and j-th row in the pixel region 702and a bias circuit 714 at around the i-th column, by using FIG. 15.

First explained is the configuration of the circuit of the pixel 713arranged at i-th column and j-th row and the bias circuit 714 at aroundthe i-th column.

The pixel of FIG. 15 has first to fourth scanning lines Ga(j)-Gd(j), asignal line S(i) and a power line V(i), and also an n-channel transistor255, a photoelectric converter element 257 and switches 250-254.

Although the transistor 255 was the n-channel type in this embodiment,the invention is not limited to this, i.e. it may be a p-channel type.However, because the transistor 255 and the transistor 260 form asource-follower circuit, the both transistors are preferably in the samepolarity.

The switches 250-254 are semiconductor elements having switchingfunctions, which preferably use transistors. The switches 251 and 252are on-off controlled according to a signal inputted through the firstscanning line Ga(j). The switch 250 is on-off controlled according to asignal inputted through the second scanning line Gb(j). The switch 253is on-off controlled according to a signal inputted through the thirdscanning line Gc(j). The switch 254 is on-off controlled according to asignal inputted through the fourth scanning line Gd(j).

The transistor 255 has source and drain regions one of which isconnected to a power line V(i) and the other is connected to a signalline S(i) through the switch 250. The transistor 255 has a gateelectrode connected to one terminal of a capacitance element 256. Theother terminal of the capacitance element 256 is connected to oneterminal of a photoelectric converter element 257 through the switch253. The other terminal of the photoelectric converter element 257 isconnected to a power line 258. The power line 258 is applied with aground potential V_(ss). The capacitance element 256 has a role to holda gate-to-source voltage of the transistor 255 during carrying out asetting operation.

The bias circuit 714 has a transistor 260, a capacitance element 261 anda switch 259. The transistor 260 has a source region connected to apower line 264 and a drain region connected to the signal line S(i). Thepower line 264 is applied with a ground potential V_(ss). The transistor260 has a gate electrode connected to one terminal of the capacitanceelement 261. The other terminal of the capacitance element 261 isconnected to the power line 264. The capacitance element 261 has a roleto hold a gate-to-source voltage of the transistor 260 during carryingout a setting operation.

247 is a reference constant-current source having a capability to flow aconstant current. The reference constant-current source 247 isconfigured by a semiconductor element such as a transistor. In thepresent specification, the reference constant-current source 247configured by a transistor will be explained in its one example inEmbodiment 6. This can be made reference to conveniently.

The power line V(i) is connected with the power line 245 through aswitch 248, and with the reference constant-current source 247 through aswitch 249. The power line 245 is applied with a power-source potentialV_(dd1) while the power line 246 is applied with a power-sourcepotential V_(dd2). The power source potential V_(dd1) applied to thepower line 245 and the power source potential V_(dd2) applied to thepower line 246 may be the same or different in value. However, the powersource potential V_(dd2) applied to the power line 246 is required to beset at a value that the reference constant-current source 247 is allowedto normally operate as a constant-current source. For example, where thereference constant-current source 247 utilizes a saturation region of atransistor to configure the current source, there is a need to set at avalue that the transistor is allowed to operate in the saturationregion.

The reference constant-current source 247 may be integrally formed witha signal-line drive circuit on a substrate. Otherwise, a constantcurrent may be inputted as a reference current externally of thesubstrate by using an IC or the like.

The arrangement position of the switches 248, 249 and referenceconstant-current source 247 is not limited to the point shown in FIG.15. Taking the foregoing Embodiments 1-3 into consideration, arrangementmay be in the different position, e.g. may be incorporated in the pixel713.

In FIG. 15, the region surrounded by the dotted line shown at 719 andregion surrounded by the dotted line shown at 714 corresponds to asource-follower circuit.

Next explained briefly is the operation of the circuit of the pixel 713arranged at i-th column and j-th row and the bias circuit 714 at aroundthe i-th column.

At first, the switches 249-252 of the pixel 713 and the switch 259 ofthe bias circuit 714 are turned into an on-state. The other switchesthan those are turned off. Thereupon, the signal current I_(data) as setin the reference constant-current source 247 flows in a direction towardthe power line 264 through the switches 249, 252, 251 and then theswitch 250 and further the switch 259.

In the instant a current begins to flow, no charge is held on thecapacitance elements 256, 261. Consequently, the transistors 255, 260are off.

Then, charge is gradually built up on the capacitance elements 256, 261to cause a potential difference at between the both electrodes of thecapacitance element 256, 261. When the potential difference at betweenthe both electrodes of the capacitance element 256, 261 reaches athreshold voltage of the transistor 255, 260, the transistors 255, 260turn on.

Then, charge storage is continued on the capacitance element 256 suchthat the gate-to-source voltage of the transistor 255 becomes a voltagecapable of flowing a predetermined signal current I_(data). Also, chargestorage is continued on the capacitance element 261 such that thegate-to-source voltage of the transistor 260 becomes a voltage capableof flowing a predetermined signal current I_(data).

After the capacitance elements 256, 261 complete the charge storage intoa steady state, the switches 251, 252, 259 are turned off. The switches249, 250 are kept on. The other switches than the above are all off. Atthis time, the signal current I_(data) set by the referenceconstant-current source 247 flows through the drain to source region ofthe transistor 255 and further the drain to source region of thetransistor 260.

Subsequently, in this state, the switches 248, 250 and 253 in the pixel713 are turned on while the other switches than those are turned off.

Thereupon, the gate electrode of the transistor 255 is inputted by asignal from the photoelectric converter element 257 through thecapacitance element 256.

At this time, the gate electrode of the transistor 255 is inputted by avalue having the signal of from the photoelectric converter element 257added onto the voltage held on the capacitance element 256. Namely, thesignal to be inputted to the gate electrode of the transistor 255 is asignal to be inputted to the gate of the same transistor in addition tothe voltage held on the capacitance element 256. Consequently, it ispossible to suppress against the affection of transistor characteristicvariation.

Then, the potential on the source region of the transistor 255 becomesan output potential V_(out). The output potential V_(out) is outputted,as a signal having been read by the photoelectric converter element 257,onto the signal line S(i) through the switch 250.

Next, the switch 254 is turned on while the other switches than thoseare turned off, to initialize the photoelectric converter element 257.More specifically, the charge held by the photoelectric converterelement 257 is allowed to flow toward the power line V(i) through theswitch 254 such that the potential on an n-channel terminal of thephotoelectric converter element 257 becomes equal to the potential onthe power line 258. From then on, the above operation is repeated.

The semiconductor device having the above configuration can suppressagainst the affection of transistor-characteristic variation.

The invention can be desirably combined with Embodiments 1-3.

[Embodiment 5]

This embodiment explains an example, different from Embodiments 3 and 4,of an electric circuit to which the invention is applied, by using FIGS.16 to 19.

In FIG. 16A, 310 is the source-follower circuit of FIGS. 1, 2A and 2B.The circuit configuration and operation of the source-follower circuit310 is similar to that of FIGS. 1, 2A and 2B, and omittedly explained inthis embodiment.

The operation of the source-follower circuit 310 is to be roughlydivided with setting and output operations, as mentioned before.Incidentally, setting operation is an operation to hold predeterminedcharge on a capacitance element, which corresponds to the operation inFIGS. 1 and 2A. Meanwhile, output operation is an operation to input aninput potential V_(in) to take out an output potential V_(out), whichcorresponds to the operation in FIG. 2B.

In the source-follower circuit 310, a terminal-a corresponds to theinput terminal while a terminal-b corresponds to the output terminal.The switches 127, 116, 118 are controlled according to a signal inputtedthrough a terminal-c. The switches 115, 117, 120 are controlledaccording to a signal inputted through a terminal-d. The switch 128 iscontrolled according to a signal inputted through a terminal-e.

In designing an electric circuit having a source-follower circuit 310,it is preferred to arrange at least two source-follower circuits 315,316 as shown in FIG. 16B. One of the source-follower circuits 315, 316is preferably to carry out a setting operation while the other is tocarry out an output operation. Because this can carry out two operationsat the same time, there is no uselessness in operation without requiringuseless time. Thus, electric circuit operation can be effected at highspeed.

In the case of an arrangement of only one source-follower circuit,output operation is not effected during setting operation. This resultsin an occurrence of useless time.

Incidentally, setting and output operations are not effected at the sametime in the source-follower circuits 315, 316. Accordingly, there is noneed to arrange one current source 126 in each of the source-followercircuits 315, 316. Namely, one current source 126 can be commonly usedby the source-follower circuits 315, 316.

For example, in a design using a source-follower circuit to asignal-line drive circuit, at least two source-follower circuits arepreferably arranged on each signal line. In a design using asource-follower circuit to a scanning-line drive circuit, at least twosource-follower circuits are preferably arranged on each scanning line.In a design using a source-follower circuit on the pixel, at least twosource-follower circuits are preferably arranged on each pixel.

In FIG. 16B, 311-314 are switches. When the switches 311, 312 are on,the switches 313, 314 are off. When the switches 311, 312 are off, theswitches 313, 314 are on. In this manner, of the two source-followercircuits 315, 316, one is cause to carry out a setting operation whilethe other is caused to carry out an output operation. Incidentally, thetwo source-follower circuits 315, 316 may be controlled by controllingthe switches 116, 118 possessed by the source-follower circuit 310without arranging the switches 311-314.

Although, in this embodiment, the region surrounded by the dotted line315, 316 was assumed corresponding to the source-follower circuit, theinvention is not limited to this, i.e. the differential amplifiercircuit, operational amplifier or the like shown in FIGS. 10-13 or thelike may be applied.

This embodiment explains the configuration and operation of asignal-line drive circuit having at least two source-follower circuitsarranged based on each signal lines, by using FIGS. 17 to 19.

FIG. 17 shows a signal-line drive circuit. The signal-line drive circuithas a sift register 321, a first latch circuit 322, a second latchcircuit 323, a D/A converter circuit 324 and a signal amplifier circuit325.

Incidentally, in the case that the first latch circuit 322 or secondlatch circuit 323 is a circuit capable of storing analog data, the D/Aconverter circuit 324 in many cases is to be omitted. In the case thatthe data to be outputted onto the signal line is binary, i.e. digitalamount, the D/A converter circuit 324 in many cases is to be omitted.Meanwhile, the D/A converter circuit 324, in a certain case,incorporates therein a gamma-correction circuit. In this manner, thesignal-line drive circuit is not limited to the configuration of FIG.17.

Briefly explaining the operation, the shift register 321 is configuredusing a plurality of columns of flip-flop circuits (FFs) or the like, toinput an input clock signal (S-CLK), a start pulse (S-SP) and a clockinversion signal (S-CLKb). Sampling pulses are to be sequentiallyoutputted according to the timing of these signals.

The sampling pulse outputted from the shift register 321 is inputted tothe first latch circuit 322. The first latch circuit 322 is inputtedwith a video signal, to hold the video signal on each column accordingto the input timing of the sampling pulse.

In the first latch circuit 322, when video-signal holding is completedto the last column, a latch pulse is inputted to the second latchcircuit 323 during a horizontal blanking period. Thus, the video signalsheld on the first latch circuit 322 are transferred, at one time, to thesecond latch circuit 323. Thereafter, the video signals held on thesecond latch circuit 323 are inputted, simultaneously in an amount ofone row, to the D/A converter circuit 324. The signal to be inputtedfrom the D/A converter circuit 324 is inputted to the signal amplifiercircuit 325.

While the video signal held on the second latch circuit 323 is beinginputted to the D/A converter circuit 324, the shift register 321 againoutputs a sampling pulse. From then on, the operation is repeated.

Explanation is made on the configuration of the signal amplifier circuit325 at around i-th column to (i+2)-th column, or three, signal lines, byusing FIG. 18.

The signal amplifier circuit 325 has two source-follower circuits 315,316 on each column. Each of the source-follower circuits 315, 316 hasfive terminals, i.e. terminal-a to terminal-e. The terminal-acorresponds to an input terminal of the source follower circuit 315, 316while the terminal-b corresponds to an output terminal of the sourcefollower circuit 315, 316. Meanwhile, the switches 127, 116, 118 arecontrolled according to a signal inputted through the terminal-c whilethe switches 115, 117, 120 are controlled according to a signal inputtedthrough the terminal-d Furthermore, the switch 128 is controlledaccording to a signal inputted through the terminal-e.

In the signal amplifier circuit 325 shown in FIG. 18, a logic operatoris arranged between the two signal lines, i.e. a setting signal line 326and a threshold signal line 327, and the source-follower circuit 315,316. 329 is an inverter, 330 is an AND, 331 and 332 are inverters, and333 is an AND. Inputted, to the terminal-c to terminal-e, is either asignal outputted from the setting signal line 327 or a signal outputtedfrom an output terminal of the logic operator.

Next explained are the signals to be outputted from the two lines, i.e.setting signal line 326 and the threshold signal line 327, and thesignals to be inputted to the switches through the terminal-c toterminal-e of the source-follower circuit 315, 316, by using FIG. 19.

Note that the switch the signal is to be inputted through the terminal-cto terminal-e is turned on when a High signal is inputted and off when aLow signal is inputted.

The signals as shown in FIG. 19 are inputted through the two signallines, i.e. the setting signal line 326 and the threshold signal line328. Furthermore, a signal outputted from the setting signal line 326 isinputted, as it is, to the terminal-c of the source-follower circuit315. A signal outputted from an output terminal of the AND 330 isinputted to the terminal-d while a signal outputted from an outputterminal of the inverter 331 is inputted to the terminal-e. By doing so,the source-follower circuit 315 can be controlled for any one of settingand outputting operations.

Also, a signal outputted from an output terminal of the inverter 332 isinputted to the terminal-c of the source-follower circuit 316. A signaloutputted from an output terminal of the AND 333 is inputted to theterminal-d while a signal outputted from the setting signal line 326 isinputted, as it is, to the terminal-e. By doing so, the source-followercircuit 316 can be controlled for any one of setting and outputtingoperations.

Incidentally, in FIGS. 16A and 16B, the current source 126 is arrangedin each source-follower circuit. Consequently, it is desired not tocause variation in value of the current flowing from the plurality ofcurrent sources 126 arranged in the signal-line drive circuit. For thisreason, it is possible not to cause variation in current value bycarrying out a setting operation to each current source 126. Thistechnique is described in the inventions of Japanese Patent ApplicationNos. 2002-287997, 2002-288104, 2002-28043, 2002-287921, 2002-287948 andso on. Accordingly, by applying this technique to the presentapplication, it is possible to correct for the characteristic variationin between the plurality of current sources 126 arranged in thesignal-line drive circuit.

Description was so far made, in FIGS. 16A, 16B, 18 and 19, on the caseusing the source-follower circuit arranged with the current sources inaddition to the native circuit as in FIG. 1. Next shown is an example ina case using a source-follower circuit as in FIG. 27 or 29.

There is shown, in FIG. 40, a diagram corresponding to FIG. 16A. Adiagram corresponding to FIG. 18 is shown in FIG. 41, while a diagramcorresponding to FIG. 19 is shown in FIG. 42. The operation or the likeis similar to those so far described and hence omitted. As compared withthe case of FIGS. 16, 18 and 19, the current source 126 is arranged inFIG. 16A whereas it is not arranged in FIG. 40. As a result, circuitarrangement is easy, making possible to carry out layout in a narrowarea. Meanwhile, as already mentioned, FIG. 16A desirably has further anadditional circuit in order not to cause variation in current valuebetween the current sources 126. This, however, is not required in thecircuit of FIG. 40. As a result, circuit arrangement is easy, makingpossible to carry out layout in a narrow area. In addition, drive timingis easier to provide.

Incidentally, the signal-line drive circuit, in many cases, has aplurality of pixels connected at the end of each signal line thereof.The pixel, in many cases, is to change its state depending upon avoltage inputted through the signal line. This may be an LCD or organicEL, for example. Besides these, connection is possible with a variety ofelements.

This embodiment can be desirably combined with Embodiments 1-4.

[Embodiment 6]

The foregoing electric circuit or semiconductor device of the inventionis arranged with a reference constant-current source having a capabilityto flow a constant current, to carry out a setting operation by the useof the reference constant-current source. The reference constant-currentsource is configured by a semiconductor element, such as a transistor.Accordingly, this embodiment explains the configuration of the referenceconstant-current source in the case of configured by a transistor and acapacitance element, by using FIGS. 20-23.

First explained is the scheme of a reference constant-current source, byusing FIGS. 20A and 20B. In FIG. 20A, 401 is a referenceconstant-current source. The reference constant-current source 401 has aterminal-A, a terminal-B and a terminal-C. The terminal-A is inputted bya setting signal. The terminal-B is supplied by a current from a currentfeed line 405. Through the terminal-C, a current set by the referenceconstant-current source 401 is supplied to the external. Namely, thereference constant-current source 401, under control of a set signalinputted to the terminal-A, is supplied with a current at the terminalB, to supply a current through the terminal-C.

In FIG. 20B, 404 is a reference constant-current source. The referenceconstant-current source 404 has a plurality of referenceconstant-current sources. It is herein assumed that there are providedtwo reference constant-current sources 402, 403. The referenceconstant-current source 402, 403 has terminals A-D. The terminal-A isinputted by a setting signal. The terminal-B is supplied with a currentfrom the current feed line 405. Through the terminal-C, a current set bythe reference constant-current source 401 is supplied to the external.The terminal D is inputted by a control signal outputted from a controlline 406. Namely, the reference constant-current source 402, 403 isunder control of a setting signal inputted at the terminal-A and acontrol signal inputted at the terminal-D, and supplied with a currentat the terminal-B to thereby supply a current at the terminal-C.

Next explained is the configuration of the reference constant-currentsource 401 of FIG. 20A, by using FIGS. 21A-21F and 22A-22E.

Each of the electric circuits shown in FIGS. 21A-21F corresponds to thereference constant-current source 401.

In FIGS. 21A and 21B, the electric circuit, having switches 54-56, ann-channel transistor 52 and a capacitance element 53 for holding agate-to-source voltage of the transistor 52 during setting operation,corresponds to the reference constant-current source 401. The electriccircuits of FIGS. 21A and 21B have the same circuit elements but aredifferent in connection relationship of between the circuit elements.

In FIG. 21C, the electric circuit, having switches 74, 75, n-channeltransistors 72, 76 and a capacitance element 73 for holding agate-to-source voltage of the transistor 72 during setting operation,corresponds to the reference constant-current source 401.

In FIGS. 21D-21F, the electric circuit, having switches 68, 70,n-channel transistors 65, 66 and a capacitance element 67 for holding agate-to-source voltage of the transistor 65, 66 during settingoperation, corresponds to the reference constant-current source 401. Theelectric circuits of FIGS. 21D-21F have the same circuit elements butare different in connection relationship of between the circuitelements.

Subsequently, explanation is briefly made on the operation of thereference constant-current source 401 of FIGS. 21A and 21B and thereference constant-current source 401 of FIGS. 21D-21F. The operation ofthe reference constant-current source 401 of FIG. 21C is similar to theoperation of FIGS. 21A and 21B, and hence omittedly explained in thisembodiment.

First explained is the operation of the reference constant-currentsource 401 of FIGS. 21A and 21B. In the electric circuit of FIGS. 21Aand 21B, the switches 54, 55 are turned on according to a signalinputted through the terminal-A. At this time, the switch 56 is off.Thereupon, a current is supplied from the current feed line 405 throughthe terminal-B, whereby predetermined charge is held on the capacitanceelement 53.

Then, the switches 54, 55 are turned off. At this time, becausepredetermined charge is held on the capacitance element 53, thetransistor 52 has a capability to flow a current in a magnitude of asignal current I_(data).

Then, the switches 54, 55 are kept in off state and the switch 56 isturned on. Thereupon, a predetermined current flows at the terminal-C.At this time, because the gate-to-source voltage of the transistor 52 ismaintained at a predetermined gate-to-source voltage, a drain currentcommensurate with the signal current I_(datra) flows through the drainregion of the transistor 52.

Incidentally, in the case of the circuit of FIGS. 21A and 21B, it isimpossible to simultaneously carry out an operation to holdpredetermined charge on the capacitance element 53 and an operation toflow a predetermined current. Consequently, controlled are the timing ofholding predetermined charge onto the capacitance element 53 and thetiming of flowing a predetermined current, by the use of the switches54-56.

Next explained is the operation of the reference constant-current source401 of FIGS. 21D-21F. In the electric circuit of FIGS. 21D-21F, theswitches 68, 70 are turned on according to a signal inputted through theterminal-A. Thereupon, a current is supplied from the current feed line405 through the terminal-B, to store predetermined charge on thecapacitance element 67. At this time, because of connection between thegate electrode of the transistor 65 and the gate electrode of thetransistor 66, the gate-to-source voltages of the transistors 65 and 66are held by the capacitance element 67.

Next, the switches 68, 70 are turned off. At this time, because ofholding predetermined charge on the capacitance element 67, thetransistor 65, 66 has a capability to flow a current in a magnitude ofsignal current I_(data). Namely, because the gate-to-source voltage ofthe transistor 66 is held at a predetermined gate-to-source voltage bythe capacitance element 67, a drain current commensurate with the signalcurrent I_(data) flows through the drain region of the transistor 66.Incidentally, in the case of the circuit of FIGS. 21D-21F, it ispossible to simultaneously carry out an operation to hold predeterminedcharge on the capacitance element 67 and an operation to flow apredetermined current.

Meanwhile, in the case of the circuit of FIGS. 21D-21F, the size of thetransistors 65, 66 is of importance. In the case the transistor 65 andthe transistor 66 are in the same size, there is a current flowingthrough the terminal-C in the same value as the current to be suppliedfrom the current feed line 405. On the other hand, where the transistor65 and the transistor 66 are different in size, i.e. when the transistor65 and the transistor 66 are different in the value of W (gate width)/L(gate length), there is a difference between the value of a currentsupplied from the current feed line 405 and the value of a currentflowing through the terminal-C. The difference relies upon the W/Lvalues of the respective transistors.

Incidentally, in the electric circuit of FIG. 21A-21F, a current isflowing from the terminal-C toward a ground potential V_(ss). FIG. 22shows a circuit configuration in a case that the transistors 52, 65, 66have a p-channel type of polarity wherein a current is flowing from theterminal-C toward a ground potential V_(ss).

Incidentally, the direction of current flow is not limited to thedirection of from the terminal-C toward the ground potential V_(ss) asshown in FIGS. 21A-21F and 22A-22E. In case the electric circuits ofFIGS. 21A-21F has a ground potential V_(ss) at the power-sourcepotential V_(dd) and further the transistors 52, 65, 66, 72 of thep-channel type, a current flows in a direction of from the power-sourcepotential V_(dd) to the terminal-C. Meanwhile, in the electric circuitof FIG. 22, in case the ground potential V_(ss) is at the power-sourcepotential V_(dd) and further the transistors 52, 65, 66 are of then-channel type, a current flows in a direction of from the power-sourcepotential V_(dd) to the terminal-C.

Next explained is the reference constant-current source 402, 403 of FIG.20B, by using FIGS. 23A and 23B. In the case of the circuit of FIG. 21Aor 21B, it was mentioned in the foregoing that simultaneous operationsare impossible between holding predetermined charge on the capacitanceelement and flowing a predetermined current. Accordingly, a plurality ofreference constant-current sources are preferably arranged as shown inFIG. 20B whereby one reference constant-current source is operated tohold predetermined charge onto the capacitance element while the otherreference constant-current source is operated to flow a predeterminedcurrent. Namely, the reference constant-current sources 402, 403 of FIG.20B preferably use the circuit of FIG. 21A or 21B.

In FIG. 23A, the circuit, having the switches 84-89, n-channeltransistor 82 and capacitance element 83 for holding a gate-to-sourcevoltage of the transistor during setting operation, corresponds to thereference constant-current source 402 or 403. The electric circuit ofFIG. 23A is the circuit of FIG. 21A or 21B.

In FIG. 23B, the circuit, having the switches 94-97, transistors 92, 98and capacitance element 93 for holding a gate-to-source voltage of thetransistor 92 during setting operation, corresponds to the referenceconstant-current source 402 or 403. The electric circuit of FIG. 23B isthe circuit of FIG. 21C.

Incidentally, the operation of the electric circuit of FIG. 23A or 23Bis similar to the operation of the electric circuit of FIG. 21A or 21B,and hence omittedly explained in this embodiment.

This embodiment can be desirably combined with Embodiment 1-5.

[Embodiment 7]

The electronic apparatus using the electric circuit of the inventionincludes a video camera, a digital camera, a goggle-type display(head-mount display), a navigation system, an audio reproducingapparatus (car audio unit, audio components, etc.), a notebook personalcomputer, a game apparatus, a personal digital assistant (mobilecomputer, cellular phone, portable game machine or electronic book), andan image reproducing apparatus having a recording medium (specifically,apparatus for reproducing a recording medium such as a Digital VersatileDisk (DVD) and having a display to display an image thereof). FIGS.4A-4H show detailed examples of these electronic apparatus.

FIG. 4A is a light-emitting apparatus including a housing 3001, asupport base 3002, a display part 3003, a speaker part 3004 and avideo-input terminal 3005. The present invention can be used in anelectric circuit configuring the display part 3003. Also, thelight-emitting apparatus of FIG. 4A can be completed by the invention.Because the light-emitting apparatus is of a spontaneous emission type,a backlight is not required. Thus, the display part can be made smallerin thickness than the liquid crystal display. Incidentally, thelight-emitting apparatus includes a display unit for displaying all thepieces of information for personal computers, TV broadcast reception,displaying advertisement and so on.

FIG. 4B is a digital still camera, including a main body 3101, a displaypart 3102, an image receiving part 3103, an operation key 3104, anexternal-connection port 3105 and a shutter 3106. The invention can beused in an electric circuit configuring the display part 3102. Also, thedigital still camera of FIG. 4B is to be completed by the invention.

FIG. 4C is a notebook personal computer, including a main body 3201, ahousing 3202, a display part 3203, a keyboard 3204, anexternal-connection port 3205 and a pointing mouse 3206. The inventioncan be used in an electric circuit configuring the display part 3203.Also, the light emitting device of FIG. 4C is to be completed by theinvention.

FIG. 4D is a mobile computer, including a main body 3301, a display part3302, a switch 3303, an operation key 3304 and an infrared ray port3305. The invention can be used in an electric circuit configuring thedisplay part 3302. Also, the mobile computer of FIG. 4D is completed bythe invention.

FIG. 4E is a portable image reproducing apparatus having a recordingmedium (specifically, DVD reproducing apparatus), including a main body3401, a housing 3402, a display part-A 3403, a display part-B 3404, arecording-medium (DVD or the like) reading part 3405, an operation key3406 and a speaker part 3407. The display part-A 3403 is to display,mainly, image information while the display part-B 3404 is to display,mainly, character information. The invention can be used in an electriccircuit configuring the display parts A and B 3403, 3404. Incidentally,the image reproducing apparatus having a recording medium includes ahome-use game apparatus. Also, the DVD reproducing apparatus of FIG. 4Eis to be completed by the invention.

FIG. 4F is a goggle-type display (head-mount display), including a mainbody 3501, a display part 3502 and an arm part 3503. The invention canbe used in an electric circuit configuring the display part 3502. Also,the goggle-type display of FIG. 4F is to be completed by the invention.

FIG. 4G is a video camera, including a main body 3601, a display part3602, a housing 3603, an external-connection port 3604, a remote-controlreceiving part 3605, an image receiving part 3606, a battery 3607, asound input part 3608, an operation key 3609, and eyepiece 3610. Theinvention can be used in an electric circuit configuring the displaypart 3602. Also, the video camera of FIG. 4G is to be completed by theinvention.

FIG. 4H is a cellular phone, including a main body 3701, a housing 3702,a display part 3703, a sound input part 3704, a sound output part 3705,an operation key 3706, an external-connection port 3707 and an antenna3708. The invention can be used in an electric circuit configuring thedisplay part 3703. Incidentally, the display part 3703 can suppress thecellular phone from consuming current by displaying white characters ona black background. Also, the cellular phone of FIG. 4H is to becompleted by the invention.

Incidentally, if light-emitting material will increase light-emissionbrightness in the future, the light containing output image informationcan be used, by magnifying and projecting by a lens or the like, on afront or rear type projector.

Meanwhile, concerning the above electronic apparatuses, there areincreasing cases to display the information distributed through anelectronic communication line, such as the Internet or CATV (cabletelevision). Particularly, there are increased occasions to displaymoving-image information. Because light-emitting material has a veryhigh response speed, the light-emitting device is preferred fordisplaying moving images.

Meanwhile, it is desired for the light-emitting device to displayinformation such that a light-emitting area is reduced to a possibleless extent because the light-emitting area consumes power. Accordingly,in the case of using a light-emitting device in a display part, mainlyfor character information, of a personal digital assistant such asparticularly a cellular phone or audio reproducing apparatus, it isdesired to carry out driving such that character information is formedby a light-emitting part with non-emitting part provided as abackground.

As described above, the present invention, having an extremely broadscope of application, can be used on an electronic apparatus in everyfield. Also, the electronic apparatus of the embodiment may use anyconfiguration of the electric circuits and semiconductor devices shownin Embodiments 1-6.

In order to cause a particular transistor to flow a current same as asignal current set in the reference constant-current source, agate-to-source voltage may be set of that transistor. In the invention,setting is possible by holding the gate-to-source voltage of thetransistor due to a capacitance element connected to that transistor. Byutilizing the voltage held on the capacitance element, it is possible tosuppress against the affection of transistor characteristic variation.

The method of utilizing a voltage held on a capacitance element can usethe method shown in the below. The voltage held on a capacitance elementis held as it is, and a signal voltage (e.g. video signal voltage) isinputted to one terminal of the capacitance element. If doing so, thegate electrode of the transistor is inputted by a voltage that thevoltage held on the capacitance element is added to the signal voltage.As a result, the gate electrode of the transistor is inputted by a valuehaving the voltage held on the capacitance element added to the signalvoltage. Namely, in the invention, even where characteristic variationoccurs between transistors, the transistor a signal voltage is to beinputted is inputted by a value that a voltage held on each capacitanceelement each transistor is connected is added to the signal voltage.Accordingly, an electric circuit can be provided that is suppressedagainst the affection of the characteristic variation betweentransistors.

What is claimed is:
 1. A display device comprising: a pixel portion overa substrate; and a source follower circuit over the substrate, thesource follower circuit comprising: a first thin film transistor; asecond thin film transistor; a first capacitor; a second capacitor; afirst switch; a second switch; a third switch; a fourth switch; a fifthswitch; and a sixth switch, wherein the first thin film transistor andthe second thin film transistor have the same polarity, wherein a gateof the first thin film transistor is directly connected to one terminalof the first capacitor and one terminal of the sixth switch, wherein theother terminal of the first capacitor is directly connected to oneterminal of the first switch and one terminal of the second switch,wherein the other terminal of the second switch is directly connected toone of a source and a drain of the first thin film transistor, one of asource and a drain of the second thin film transistor, one terminal ofthe third switch, and one terminal of the fourth switch, wherein theother of the source and the drain of the first thin film transistor isdirectly connected to one terminal of the fifth switch, wherein theother terminal of the fourth switch is directly connected to a gate ofthe second thin film transistor and one terminal of the secondcapacitor, wherein the other terminal of the second capacitor isdirectly connected to the other of the source and the drain of thesecond thin film transistor, wherein the other of the source and thedrain of the second thin film transistor is directly connected to afirst power supply line, wherein the other terminal of the fifth switchis directly connected to a second power supply line, wherein the otherterminal of the first switch is directly connected to an input terminal,wherein the other terminal of the third switch is directly connected toan output terminal, wherein the pixel portion comprises a third thinfilm transistor and a display element, wherein the third thin filmtransistor is electrically connected to the display element, and whereinthe source follower circuit is operationally connected to the pixelportion.
 2. The display device according to claim 1, wherein the otherterminal of the sixth switch is directly connected to the other of thesource and the drain of the first thin film transistor.
 3. The displaydevice according to claim 1, further comprising a seventh switch,wherein one terminal of the seventh switch is directly connected to theother of the source and the drain of the first thin film transistor. 4.The display device according to claim 1, further comprising a thirdpower supply line electrically connected to the other of the source andthe drain of the first thin film transistor.
 5. The display deviceaccording to claim 1, further comprising a current source electricallyconnected to the other of the source and the drain of the first thinfilm transistor.
 6. The display device according to claim 1, furthercomprising: a seventh switch; and a third power supply line, wherein oneterminal of the seventh switch is electrically connected to the other ofthe source and the drain of the first thin film transistor, and whereinthe other terminal of the seventh switch is electrically connected tothe third power supply line.
 7. The display device according to claim 1,further comprising: a seventh switch; and a current source, wherein oneterminal of the seventh switch is directly connected to the other of thesource and the drain of the first thin film transistor, and wherein theother terminal of the seventh switch is directly connected to thecurrent source.
 8. The display device according to claim 1, furthercomprising: a pixel portion over a substrate; and a source followercircuit over the substrate, the source follower circuit comprising: afirst thin film transistor; a second thin film transistor; a firstcapacitor; a second capacitor; a first switch; a second switch; a thirdswitch; a fourth switch; a fifth switch; and a sixth switch, wherein thefirst thin film transistor and the second thin film transistor have thesame polarity, wherein a gate of the first thin film transistor isdirectly connected to one terminal of the first capacitor and oneterminal of the sixth switch, wherein the other terminal of the firstcapacitor is directly connected to one terminal of the first switch andone terminal of the second switch, wherein the other terminal of thesecond switch is directly connected to one of a source and a drain ofthe first thin film transistor, one of a source and a drain of thesecond thin film transistor, one terminal of the third switch, and oneterminal of the fourth switch, wherein the other of the source and thedrain of the first thin film transistor is directly connected to oneterminal of the fifth switch, wherein the other terminal of the fourthswitch is directly connected to a gate of the second thin filmtransistor and one terminal of the second capacitor, a current source;and a third power supply line, wherein one terminal of the currentsource is electrically connected to the other of the source and thedrain of the first thin film transistor, and wherein the other terminalof the current source is electrically connected to the third powersupply line.
 9. The display device according to claim 1, furthercomprising: a seventh switch; a current source; and a third power supplyline, wherein one terminal of the seventh switch is directly connectedto the other of the source and the drain of the first thin filmtransistor, wherein the other terminal of the seventh switch is directlyconnected to one terminal of the current source, and wherein the otherterminal of the current source is directly connected to the third powersupply line.
 10. A display module comprising: a FPC; and the displaydevice according to claim
 1. 11. An electronic apparatus comprising: thedisplay module according to claim 10; and at least one of a speaker, animage receiving part, an external connection port, a battery, a soundinput part, and an antenna.
 12. A display device comprising: wherein theother terminal of the second capacitor is directly connected to theother of the source and the drain of the second thin film transistor,wherein the other of the source and the drain of the second thin filmtransistor is directly connected to a first power supply line, whereinthe other terminal of the fifth switch is directly connected to a secondpower supply line, wherein the other terminal of the first switch isdirectly connected to an input terminal, wherein the other terminal ofthe third switch is directly connected to an output terminal, whereinthe pixel portion comprises a third thin film transistor and a displayelement, wherein the third thin film transistor is electricallyconnected to the display element, wherein the source follower circuit isoperationally connected to the pixel portion, and wherein the firstswitch, the second switch, the third switch, the fourth switch, thefifth switch, and the sixth switch are each a thin film transistor. 13.The display device according to claim 12, wherein the other terminal ofthe sixth switch is directly connected to the other of the source andthe drain of the first thin film transistor.
 14. The display deviceaccording to claim 12, further comprising a seventh switch, wherein oneterminal of the seventh switch is directly connected to the other of thesource and the drain of the first thin film transistor.
 15. The displaydevice according to claim 12, further comprising a third power supplyline electrically connected to the other of the source and the drain ofthe first thin film transistor.
 16. The display device according toclaim 12, further comprising a current source electrically connected tothe other of the source and the drain of the first thin film transistor.17. The display device according to claim 12, further comprising: aseventh switch; and a third power supply line, wherein one terminal ofthe seventh switch is electrically connected to the other of the sourceand the drain of the first thin film transistor, and wherein the otherterminal of the seventh switch is electrically connected to the thirdpower supply line.
 18. The display device according to claim 12, furthercomprising: a seventh switch; and a current source, wherein one terminalof the seventh switch is directly connected to the other of the sourceand the drain of the first thin film transistor, and wherein the otherterminal of the seventh switch is directly connected to the currentsource.
 19. The display device according to claim 12, furthercomprising: a current source; and a third power supply line, wherein oneterminal of the current source is electrically connected to the other ofthe source and the drain of the first thin film transistor, and whereinthe other terminal of the current source is electrically connected tothe third power supply line.
 20. The display device according to claim12, further comprising: a seventh switch; a current source; and a thirdpower supply line, wherein one terminal of the seventh switch isdirectly connected to the other of the source and the drain of the firstthin film transistor, wherein the other terminal of the seventh switchis directly connected to one terminal of the current source, and whereinthe other terminal of the current source is directly connected to thethird power supply line.
 21. A display module comprising: a FPC; and thedisplay device according to claim
 12. 22. An electronic apparatuscomprising: the display module according to claim 21; and at least oneof a speaker, an image receiving part, an external connection port, abattery, a sound input part, and an antenna.
 23. A display devicecomprising: a pixel portion over a substrate; and a source followercircuit over the substrate, the source follower circuit comprising: afirst thin film transistor; a second thin film transistor; a firstcapacitor; a second capacitor; a first switch; a second switch; a thirdswitch; a fourth switch; a fifth switch; a sixth switch; and a seventhswitch, wherein the first thin film transistor and the second thin filmtransistor have the same polarity, wherein a gate of the first thin filmtransistor is directly connected to one terminal of the first capacitorand one terminal of the sixth switch, wherein the other terminal of thefirst capacitor is directly connected to one terminal of the firstswitch and one terminal of the second switch, wherein the other terminalof the second switch is directly connected to one of a source and adrain of the first thin film transistor, one of a source and a drain ofthe second thin film transistor, and one terminal of the fourth switch,wherein the other of the source and the drain of the first thin filmtransistor is directly connected to one terminal of the fifth switch,wherein the other terminal of the fourth switch is directly connected toa gate of the second thin film transistor and one terminal of the secondcapacitor, wherein the other terminal of the second capacitor isdirectly connected to the other of the source and the drain of thesecond thin film transistor, wherein the other of the source and thedrain of the second thin film transistor is directly connected to afirst power supply line, wherein one terminal of the third switch isdirectly connected to one terminal of the seventh switch, wherein theother terminal of the first switch is directly connected to an inputterminal, wherein the other terminal of the third switch is directlyconnected to an output terminal, wherein the pixel portion comprises athird thin film transistor and a display element, wherein the third thinfilm transistor is electrically connected to the display element, andwherein the source follower circuit is operationally connected to thepixel portion.
 24. The display device according to claim 23, wherein theother terminal of the sixth switch is directly connected to the other ofthe source and the drain of the first thin film transistor.
 25. Thedisplay device according to claim 23, further comprising an eighthswitch, wherein one terminal of the eighth switch is directly connectedto the other of the source and the drain of the first thin filmtransistor.
 26. The display device according to claim 23, furthercomprising a third power supply line electrically connected to the otherof the source and the drain of the first thin film transistor.
 27. Thedisplay device according to claim 23, further comprising a currentsource electrically connected to the other of the source and the drainof the first thin film transistor.
 28. The display device according toclaim 23, further comprising: an eighth switch; and a third power supplyline, wherein one terminal of the eighth switch is electricallyconnected to the other of the source and the drain of the first thinfilm transistor, and wherein the other terminal of the eighth switch iselectrically connected to the third power supply line.
 29. The displaydevice according to claim 23, further comprising: an eighth switch; anda current source, wherein one terminal of the eighth switch is directlyconnected to the other of the source and the drain of the first thinfilm transistor, and wherein the other terminal of the eighth switch isdirectly connected to the current source.
 30. The display deviceaccording to claim 23, further comprising: a current source; and a thirdpower supply line, wherein one terminal of the current source iselectrically connected to the other of the source and the drain of thefirst thin film transistor, and wherein the other terminal of thecurrent source is electrically connected to the third power supply line.31. The display device according to claim 23, further comprising: aneighth switch; a current source; and a third power supply line, whereinone terminal of the eighth switch is directly connected to the other ofthe source and the drain of the first thin film transistor, wherein theother terminal of the eighth switch is directly connected to oneterminal of the current source, and wherein the other terminal of thecurrent source is directly connected to the third power supply line. 32.The display device according to claim 23, wherein the other terminal ofthe fifth switch is electrically connected to a second power supplyline.
 33. The display device according to claim 23, wherein the otherterminal of the seventh switch is electrically connected to a secondpower supply line.
 34. The display device according to claim 23, furthercomprising a fourth thin film transistor, wherein one of a source and adrain of the fourth thin film transistor is directly connected to asecond power supply line, and wherein the other of the source and thedrain of the fourth thin film transistor is directly connected to theother terminal of the fifth switch.
 35. The display device according toclaim 23, further comprising a fifth thin film transistor, wherein oneof a source and a drain of the fifth thin film transistor is directlyconnected to a second power supply line, and wherein the other of thesource and the drain of the fifth thin film transistor is directlyconnected to the other terminal of the seventh switch.
 36. The displaydevice according to claim 23, further comprising a sixth thin filmtransistor, wherein one of a source and a drain of the sixth thin filmtransistor is directly connected to the one terminal of the seventhswitch, and wherein the other of the source and the drain of the sixththin film transistor is directly connected to the other terminal of thesecond switch.
 37. A display module comprising: a FPC; and the displaydevice according to claim
 23. 38. An electronic apparatus comprising:the display module according to claim 37; and at least one of a speaker,an image receiving part, an external connection port, a battery, a soundinput part, and an antenna.
 39. A display device comprising: a pixelportion over a substrate; and a source follower circuit over thesubstrate, the source follower circuit comprising: a first thin filmtransistor; a second thin film transistor; a first capacitor; a secondcapacitor; a first switch; a second switch; a third switch; a fourthswitch; a fifth switch; a sixth switch; and a seventh switch, whereinthe first thin film transistor and the second thin film transistor havethe same polarity, wherein a gate of the first thin film transistor isdirectly connected to one terminal of the first capacitor and oneterminal of the sixth switch, wherein the other terminal of the firstcapacitor is directly connected to one terminal of the first switch andone terminal of the second switch, wherein the other terminal of thesecond switch is directly connected to one of a source and a drain ofthe first thin film transistor, one of a source and a drain of thesecond thin film transistor, and one terminal of the fourth switch,wherein the other of the source and the drain of the first thin filmtransistor is directly connected to one terminal of the fifth switch,wherein the other terminal of the fourth switch is directly connected toa gate of the second thin film transistor and one terminal of the secondcapacitor, wherein the other terminal of the second capacitor isdirectly connected to the other of the source and the drain of thesecond thin film transistor, wherein the other of the source and thedrain of the second thin film transistor is directly connected to afirst power supply line, wherein one terminal of the third switch isdirectly connected to one terminal of the seventh switch, wherein theother terminal of the first switch is directly connected to an inputterminal, wherein the other terminal of the third switch is directlyconnected to an output terminal, wherein the pixel portion comprises athird thin film transistor and a display element, wherein the third thinfilm transistor is electrically connected to the display element,wherein the source follower circuit is operationally connected to thepixel portion, and wherein the first switch, the second switch, thethird switch, the fourth switch, the fifth switch, the sixth switch, andthe seventh switch are each a thin film transistor.
 40. The displaydevice according to claim 39, wherein the other terminal of the sixthswitch is directly connected to the other of the source and the drain ofthe first thin film transistor.
 41. The display device according toclaim 39, further comprising an eighth switch, wherein one terminal ofthe eighth switch is directly connected to the other of the source andthe drain of the first thin film transistor.
 42. The display deviceaccording to claim 39, further comprising a third power supply lineelectrically connected to the other of the source and the drain of thefirst thin film transistor.
 43. The display device according to claim39, further comprising a current source electrically connected to theother of the source and the drain of the first thin film transistor. 44.The display device according to claim 39, further comprising: an eighthswitch; and a third power supply line, wherein one terminal of theeighth switch is electrically connected to the other of the source andthe drain of the first thin film transistor, and wherein the otherterminal of the eighth switch is electrically connected to the thirdpower supply line.
 45. The display device according to claim 39, furthercomprising: an eighth switch; and a current source, wherein one terminalof the eighth switch is directly connected to the other of the sourceand the drain of the first thin film transistor, and wherein the otherterminal of the eighth switch is directly connected to the currentsource.
 46. The display device according to claim 39, furthercomprising: a current source; and a third power supply line, wherein oneterminal of the current source is electrically connected to the other ofthe source and the drain of the first thin film transistor, and whereinthe other terminal of the current source is electrically connected tothe third power supply line.
 47. The display device according to claim39, further comprising: an eighth switch; a current source; and a thirdpower supply line, wherein one terminal of the eighth switch is directlyconnected to the other of the source and the drain of the first thinfilm transistor, wherein the other terminal of the eighth switch isdirectly connected to one terminal of the current source, and whereinthe other terminal of the current source is directly connected to thethird power supply line.
 48. The display device according to claim 39,wherein the other terminal of the fifth switch is electrically connectedto a second power supply line.
 49. The display device according to claim39, wherein the other terminal of the seventh switch is electricallyconnected to a second power supply line.
 50. The display deviceaccording to claim 39, further comprising a fourth thin film transistor,wherein one of a source and a drain of the fourth thin film transistoris directly connected to a second power supply line, and wherein theother of the source and the drain of the fourth thin film transistor isdirectly connected to the other terminal of the fifth switch.
 51. Thedisplay device according to claim 39, further comprising a fifth thinfilm transistor, wherein one of a source and a drain of the fifth thinfilm transistor is directly connected to a second power supply line, andwherein the other of the source and the drain of the fifth thin filmtransistor is directly connected to the other terminal of the seventhswitch.
 52. The display device according to claim 39, further comprisinga sixth thin film transistor, wherein one of a source and a drain of thesixth thin film transistor is directly connected to the one terminal ofthe seventh switch, and wherein the other of the source and the drain ofthe sixth thin film transistor is directly connected to the otherterminal of the second switch.
 53. A display module comprising: a FPC;and the display device according to claim
 39. 54. An electronicapparatus comprising: the display module according to claim 53; and atleast one of a speaker, an image receiving part, an external connectionport, a battery, a sound input part, and an antenna.